• Preheat the oven to 350 degrees F. Grease and flour two 9×2 inch round cake pans and set them on a baking sheet. You can also use 8x2 inch round cake pans, but the bake time will change.
• 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, until golden and springy to the touch and a thin knife inserted comes 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.

Espresso Extract Mixture

• To make the extract, stir the espresso powder and boiling water together in a small cup until blended. Set aside.

Espresso Syrup Mixture

• To make the syrup, stir the water and sugar together in a small saucepan and bring just to a boil. Pour the syrup into a small heatproof bowl and stir in 1 tablespoon of the espresso extract and the liqueur; set aside.

Filling and Frosting

• To make the filling and frosting, put the mascarpone, sugar, vanilla, and liqueur in a large bowl and whisk just until blended and smooth.
• In another mixing bowl using the whisk attachment of a stand mixer or a handheld mixer, whip the heavy cream until it holds firm peaks. Stir about one quarter of the whipped cream into the mascarpone. With a rubber scraper, fold in the rest of the whipped cream.

Assemble 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. You can do this by spooning it over and gently brushing.
• Smooth some of the mascarpone cream over the layer - user about 1 ¼ cups - and gently press the chopped chocolate into the mascarpone mixture. Turn the second layer of the cake upside down and soak with more espresso syrup by spooning it over. You can use all or less if desired. Stack over first layer.
• For the frosting, whisk 1 to 1 ½ 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.
• To garnish, you can sprinkle the cake with cocoa powder and garnish with chocolate espresso beans or pipe or spoon dollops of whipped cream (or just use whipped topping) over the top and then sprinkle with more cocoa powder.
• 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.