Center a rack in the oven and preheat the oven to 350 degrees F. Butter two 9 x 2 inch round cake pans, dust the insides with flour, tap out the excess and line the bottoms of the pans with parchment or wax paper. Put the pans on a baking sheet.
To make the cake:
Sift together the cake flour, baking powder, baking soda, and salt. Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed 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; don’t be concerned if the mixture looks curdled. Reduce the mixer speed to low and add the dry ingredients alternately with the buttermilk, adding the dry ingredients in three additions and the buttermilk in two (begin and end with the dry ingredients); 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 28-30 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 them 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. Pour the syrup into a small heatproof bowl and stir 1 tablespoon of the espresso extract and the liqueur or brandy; set aside.
To make the filling and frosting:
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.
Make 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 , 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 them.
With a long metal icing spatula, smooth the frosting around the sides of the cake and over the top.
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, or top with shaved chocolate.
Optional: decorate the cake with chocolate covered espresso beans.
For the best appearance, don't add the dusting of cocoa until you are ready to serve.
This cake can be made 1 to 2 days ahead and stored in the refrigerator.
Tiramisu desserts specifically mention espresso, not regular coffee. If you must use coffee, strongly-brewed coffee will work in a traditional cake. However, for this cake it would be better to use instant espresso powder.
To freeze, after baking the cake, frost it but don't dust it with cocoa. Once the cake has chilled in the refrigerator for at least 4 hours, you can put it in the freezer for up to 2 months.
Can I use cream cheese? You will not get the same result with regular cream cheese, however, you can try beating together 8 ounces of cream cheese (full fat, not low fat) with 4 tablespoons of heavy whipping cream and 2 tablespoons of softened unsalted butter until just blended. This will give you the equivalent of approximately 10 ounces (1 1/4 cups) mascarpone.
Can I replace mascarpone with ricotta? While the flavor of this substitute is not exactly the same as real mascarpone, it works very well in many recipes. Ricotta cheese can be a low-cost substitute for mascarpone.