Preheat the oven to 350 degrees F. Butter two 8" round cake pans and line the bottoms with baking parchment.
You can use an electric mixer with a beater attachment, a food processor, or a bowl and an electric whisk. First, sift the flour and baking powder into the bowl, then add the butter, sugar, eggs, and coffee. Beat together until well blended, but be very careful not to overmix, as you want a light cake. Divide the batter evenly between the pans and level the tops.
Bake for about 25 minutes, or until a skewer emerges clean. Remove from the oven and leave for a couple of minutes. Run a knife around the rims to loosen the cakes from the pans and turn out onto a wire rack. Remove the papers and let cool completely.
To make the filling, tip the mascarpone into a bowl and lightly beat. Fold in the heavy cream, sugar, and vanilla. Chill until you are ready to assemble the cake.
Split the two cakes in half horizontally, to make four layers. In a little bowl, mix together the coffee and Tia Maria.
To assemble the cake, place one layer of cake on a plate. Sprinkle the coffee mixture over all four layers. Take about a quarter of the mascarpone filling and spread it over the base cake, dust with cocoa, and place the second layer on top. Repeat until you have a four-layer cake, topped with the mascarpone filling. Sprinkle the grated chocolate over the top and decorate with the chocolate-covered coffee beans. Chill for an hour or two until ready to serve.
This tiramisu cake recipe is excerpted with permission from <a href="http://www.amazon.com/gp/product/0847838757/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&tag=craftfocom-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=0847838757">"Birthday Cakes: Festive Cakes for Celebrating that Special Day"</a> by Fiona Cairns and published by Rizzoli.