Chocolate Chestnut Cake

Ingredients: 

Chocolate Cake
180 grams (1½ cups) all-purpose flour
60 grams (½ cup) medium rye flour, finely milled (substitute all-purpose flour if not available)
1 teaspoon baking soda
1 teaspoon smoked salt (substitute kosher salt if not accessible)
177 mL (¾ cup) extra virgin olive oil
300 grams (1½ cups) granulated sugar
35 grams (⅓ cup) Dutch process cocoa powder, sifted
250 mL (1 cup) hot coffee (or hot water/smoky black tea such as lapsang souchong)
2 large eggs
120 grams (½ cup) sour cream, full fat
1½ teaspoons vanilla extract

Chestnut Chocolate Mousse
300 grams (10.4 ounces, about 2 cups) chestnuts, roasted and peeled (vacuum-packed, steamed, or whole roasted in jar)
500 mL (2 cups) whole milk
100 grams (½ cup) granulated sugar
1 Tablespoon vanilla extract
½ teaspoon smoked salt (substitute kosher salt if not accessible)
113 grams (4 ounces, ⅔ cup chopped) dark chocolate, finely chopped (I used a 72% chocolate bar)
500 mL (2 cups) heavy whipping cream, chilled

Chocolate Ganache
113 grams (4 ounces, ⅔ cup chopped) dark chocolate, finely chopped (I used a 72% chocolate bar)
165 mL (⅔ cup) heavy whipping cream
Chocolate shavings or chocolate baking pearls for garnish

Instructions

Chocolate Cake
Preheat oven to 325° F (165° C, Gas Mark 3). Place oven rack in center of oven. Prepare a half sheet baking pan (18 by 13 by 1 inch; 457 by 330 by 25 centimeters) by covering the entire base of the pan with a piece of parchment paper. DO NOT grease the pan.

In a medium bowl, combine flours, baking soda, and smoked salt. Stir with a whisk to blend well. Set aside.

In a large mixing bowl, whisk together olive oil, sugar, cocoa powder, and hot coffee. Add eggs one at a time, whisking well between additions to ensure they are fully incorporated.

Add dry ingredients and whisk again. Scrape the bowl with a spatula to ensure no flour pockets remain. Add sour cream and vanilla and whisk until completely blended and batter is smooth. Pour batter into prepared half sheet pan. Rap pan firmly on countertop several times to remove air bubbles that may have formed in batter. Place pan in center rack of oven and bake for 15-18 minutes, until a toothpick inserted in the middle comes out clean, and the middle springs back when touched.

Cool cake for 15 minutes on a baking rack, then carefully run a sharp knife around the cake edges to loosen it from the pan. To flip the cake out of the pan, cover the top of the cake with a piece of parchment paper. Gently place another half sheet pan on top of the parchment covered cake and flip it over. Give it a little rap on the counter, and then gently remove the top pan. Peel off the parchment paper and let cake cool completely.

Chestnut Chocolate Mousse
Using your fingers (or a knife), break up the whole chestnuts into small pieces. Place them in a 3-quart saucepan along with the milk, sugar, vanilla and salt. Bring the mixture to a boil, then reduce the heat to a simmer and cook, stirring from time to time, until chestnuts have softened, and liquid is reduced by about a quarter, 15-20 minutes.

Place mixture into the bowl of a blender. Blend at low speed, then increase speed to high and blend until chestnut mixture is silky smooth and fully pureed. Add the finely chopped chocolate and blend again. Scrape down the sides of the blender to incorporate all chocolate. Once chocolate is fully blended in, pour chocolate chestnut puree mixture into a wide bowl to cool to room temperature. Whisk the mixture from time to time to encourage the cooling process. If it begins to thicken or form lumps, whisking should bring it back to a smooth consistency.

In the bowl of a stand mixer fitted with the whisk attachment, beat the heavy cream on high speed until soft peaks form. Fold ⅓ of the whipped cream into the cooled chestnut chocolate mixture. Once incorporated, fold in the remaining whipped cream, being careful not to overmix.

Cake Assembly
With a large offset spatula, spread chestnut chocolate mousse onto the cooled sheet cake (which is still resting on a piece of parchment on a baking sheet). Work to make this mousse layer as level as possible. This will be important when you stack the layers later. Extend the mousse to the edges of the cake. Place into fridge to set for at least 3 hours. If you will leave it for more than 3 hours, cover with plastic wrap to keep fresh.

Once mousse is set, remove cake from fridge and slide off of baking pan onto a large cutting board or work surface. Using a large knife, trim about ½ inch (1.27 cm) off of each side to achieve clean, tidy lines and a close-to-perfect rectangle shape. If helpful, use a bit of kitchen twine or dental floss to gently mark a straight line on the top of the mousse before cutting. Wipe the knife clean between each cut to help ensure tidy cake layers.

Mark the center of the cake both lengthwise and width wise. Cut the cake in half lengthwise, then width wise, again cleaning the knife between each cut. You should end up with four equal rectangles.

Run the tip of a paring knife along the cuts to make sure the parchment paper under the cake has also been cut. Place a serving platter nearby. Lift the first rectangle of cake/mousse and gently peel the parchment paper down and away from one end of the cake. Place that side of the cake on the platter and pull the rest of the parchment away as you carefully set down the cake rectangle. Keep the cake as flat as possible to avoid cracking.

Repeat the cake transfer and parchment paper removal process with the remaining 3 layers of cake, stacking each onto the next. Make sure the cake is positioned accurately before placing it down onto the layer below it. Once it is placed, it's hard to shift the layers much. Once all four layers have been stacked, place into fridge to cool while you make the chocolate ganache.

Chocolate Ganache
In a two-quart pot, heat cream until almost simmering. Remove from heat and add finely chopped dark chocolate. Let sit for 1 minute, and then gently whisk until all chocolate has melted and no lumps remain.

Pour chocolate ganache onto top of cake. Using a mini offset spatula or the back of a spoon, spread the ganache to the edges of the cake but do not let it drip down the sides. Garnish with chocolate shavings or chocolate baking pearls.

Cake should be refrigerated until served. The cake also slices best when the mousse is firm. Store any extra cake in an airtight container (or wrap in plastic wrap) in the refrigerator.