Preheat oven to 350 degrees Fahrenheit. Grease 9x13 inch pan.

In a medium bowl, sift together flour, baking powder and salt. Set aside.

In a small saucepan, combine butter and milk over medium heat just until butter is melted. Do not let it boil. Remove from heat and set aside.

In a stand mixer use the whisk attachment, or use a hand mixer, whisk eggs on high speed until combined and starting to foam, about a minute. Gradually add sugar and keep whisking until tripled in volume, about 5 minutes. Beat in vanilla.

At medium speed add warm milk mixture into the egg mixture in a slow steady stream. Add flour mixture gradually and whisk until combined.

Pour batter into prepared cake pan and bake 25-35 minutes or until toothpick comes out clean. Remove to cool. While cake is still warm use fork or toothpick and poke lots of holes in the cake.

Make syrup by stirring hot coffee and sugar until sugar is dissolved, then add your liqueur if using.

Brush the coffee mixture over the cake gradually,allowing it to soak in. Use all the syrup.

Make the frosting: Using your whisk attachment, whisk the mascarpone, sugar and liqueur (if using) on medium speed. Gradually add heavy cream and increase speed to high. Whisk until stiff peaks form and frosting is thick. Immediately frost cake. Just before serving dust with cocoa powder.

This will keep in refrigerator up to 4 days.