Turn a simple box mix into an elegant Tiramisu-inspired cake with hints of espresso, chocolate, and rum. This cake is frosted "naked style," for a trendy rustic dessert that requires no special decorating skills.
Preheat oven to 350°F. Grease two 8" round baking pans.
In a large bowl, whisk together milk, rum, espresso powder, oil, eggs, and egg yolk until smooth. Add the cake mix and beat until smooth. Divide into two prepared pans and bake about 30 to 35 minutes, or until a tester inserted in center comes out clean. Cool about 10 minutes in pan, transfer to cooling racks, and pour ¼ cup of rum evenly over each layer.
To make frosting, combine mascarpone cheese, cream cheese, butter, and vanilla in the base of a mixer and beat until smooth. Beat in confectioners sugar about ½ cup at a time until fully incorporated, light and fluffy.
Slice completely cooled cake layers down center to create four even layers.
To assemble cake, place one cake layer on a cake plate and top with ¼ frosting, spreading it just until it reaches slightly to the edge. Layer with another layer of cake and another layer of frosting, repeating until you finish with a final layer of frosting. Dust with cocoa powder and finish with chocolate shavings.