Prepare the mascarpone and whipped cream at least one day in advance and refrigerate. Bake vanilla cake in a 9x13 prepared baking dish. I bake the cake a little longer to make sure it's nice and firm. Leave it in the dish to cool.
Filling
* Add the mascarpone, whipped cream, vanilla, 2 oz of rum, and 2 oz of espresso.
* Beat with electric whisk until creamy
Finish
* Mix the rest of the rum and coffee
* Pour evenly over cooled cake
* Spread the filling over cake
* Sift the cocoa over the top
* Refrigerate until firm
* Cut and enjoy