Tiramisu Cake

Preheat the oven to 350 degrees F. Grease and flour two 92 inch round cake pans and set them on a baking sheet.Sift together the cake flour, baking powder, baking soda, and salt.Beat the butter on medium speed of an electric mixer until soft and creamy.

Add the sugar and beat for another 3 minutes.

Add the eggs one by one, and then the yolk, beating for 1 minute after each addition. Beat in the vanilla; Reduce the mixer speed to low and add the dry ingredients alternately with the buttermilk; scrape down the sides of the bowl as needed and mix only until the ingredients disappear into the batter. Divide the batter evenly between the two pans and smooth the tops with a rubber spatula.

Bake for 28 to 30 minutes, rotating the pans at the midway point. When fully baked, the cakes will be golden and springy to the touch and a thin knife inserted into the centers will come out clean.

Transfer the cakes to a rack and cool for about 5 minutes, then run a knife around the sides of the cakes, unmold them, and peel off the paper liners. Invert and cool to room temperature right-side up.To make the extract:Stir the espresso powder and boiling water together in a small cup until blended. Set aside.To make the syrup:Stir the water and sugar together in a small saucepan and bring just to a boil.

Put the mascarpone, sugar, vanilla, and liqueur in a large bowl and whisk just until blended and smooth.Working with the stand mixer with the whisk attachment or with a hand mixer, whip the heavy cream until it holds firm peaks. Switch to a rubber spatula and stir about one quarter of the whipped cream into the mascarpone. Fold in the rest of the whipped cream with a light touch.To assemble the cake:If the tops of the cake layers have crowned, use a long serrated knife and a gentle sawing motion to even them.

Place one layer right-side up on a cardboard round or a cake plate protected with strips of wax or parchment paper. Using a pastry brush or a small spoon, soak the layer with about one third of the espresso syrup. Smooth some of the mascarpone cream over the layer - user about 1 1/4 cups - and gently press the chopped chocolate into the filling.

Put the second cake layer on the counter and soak the top of it with half the remaining espresso syrup, then turn the layer over and position it, soaked side down, over the filling. Soak the top of the cake with the remaining syrup.For the frosting, whisk 1 to 1 1/2 tablespoons of the remaining espresso extract into the remaining mascarpone filling. Taste the frosting as you go to decide how much extract you want to add. If the frosting looks as if it might be a little too soft to spread over the cake, press a piece of plastic wrap against its surface and refrigerate it for 15 minutes or so. Refrigerate the cake too.With a long metal icing spatula, smooth the frosting around the sides of the cake and over the top. If you want to decorate the cake with chocolate-covered espresso beans, press them into the filling, making concentric circles of beans or just putting some beans in the center of the cake.Refrigerate the cake for at least 3 hours (or for up to 1 day) before serving - the elements need time to meld.Just before serving, dust the top of the cake with cocoa.