Prepare the cake mix as instructed on the box. Add 3 tsp instant decaf coffee to the cake batter and mix.
Divide evenly into 3 greased 9 inch cake pans. Bake 20-25 minutes at 350F. Let cool completely.
Combine the remaining 1 t instant decaf coffee with ¼ c water and 1 T Kahlua. Set aside.
Filling
Beat the mascarpone cheese, ½ c powdered sugar and 2 Tbsp Kahlua until smooth and creamy. Refrigerate.
Frosting
Whip 2 c heavy whipping cream, ¼ c powdered sugar, and 1 t vanilla until peaks form. Add ½ c of the frosting to the filling and mix.
Refrigerate. Place the first cake on a cake stand or whatever plate you will serve it on. Poke holes with a meat fork and brush reserved coffee mix over the cake.
Spread half of the filling mix on top. Repeat with the next two layers.
Pour any remaining coffee mix over the top layer of cake. Spread frosting over entire cake. Garnish with semi-sweet chocolate or dust with cocoa as desired. Refrigerate for at least 30 minutes prior to serving.