For the filling:
1 cup heavy cream, cold
1/2 cup powdered sugar
1 tsp vanilla bean paste
1 8oz container mascarpone cheese, cold
For the coffee soak:
1/4 cup (60ml) strongly brewed espresso
Cocoa powder to dust
Make the cakes:
Preheat the oven to 350˚F. Grease and line two 6 inch cake tins. Prepare the Chocolate mix as directed on the box. Divide evenly between the two pans using a scale. Bake for 20 minutes, or until a skewer comes out clean. Cool in the tins for 10 minutes before turning out onto a wire rack and cooling completely.
Repeat with the vanilla mix. Cool completely.
Level off the cakes so that they have flat tops.
In the bowl of a stand mixer fitted with the whisk attachment, or using an electric mixer, whip the cream, powdered sugar and vanilla together until soft peaks form. Add the mascarpone and whip until stiff peaks form. Transfer to a bowl.
Place the chocolate frosting in the bowl of a stand mixer fitted with the paddle attachment. Whip the frosting on medium high for 2-3 minutes, until silky and smooth. Transfer about 1/4 to a piping bag fitted with a round tip. Transfer the rest to a bowl. Wash and dry the bowl. Repeat the whipping process with the vanilla frosting. Transfer a small amount to a piping bag fitted with a french star attachment, and transfer the remainder to a bowl.
Place a chocolate layer on a turntable or cake stand. Brush liberally with the espresso. Pipe a 'dam' of chocolate frosting around the outside of the layer. Fill the dam with the mascarpone frosting, and smooth off with a small offset spatula. Top with a vanilla layer of cake. Brush with espresso, pipe a chocolate dam, and fill with mascarpone filling. Repeat with a chocolate layer, then top off with a layer of vanilla cake.
Using an offset spatula, spread a layer of chocolate frosting around the bottom half of the cake. Spread a layer of Vanilla frosting over the top of the cake and around the top half of the cake. Using the offset spatula, blend where the two flavored frostings meet slightly. Smooth the sides of the cake using an icing scraper. Patch up any holes or bare patches if needed, and smooth again. Smooth the top edge using the offset spatula, wiping between each pass. Finish the cake off by piping a ring of vanilla frosting 'blobs' around the top of the cake, using the french star tip. Top with a light dusting of cocoa, if desired.