Briskly stir 3 tablespoons of the rum into the Buttercream with a rubber spatula. In a separate bowl, combine the walnuts with ⅔ cup of the rum buttercream. Combine the remaining rum with the Simple Syrup.
Using a serrated knife, cut Génoise, horizontally, into 3 layers. Invert top layer onto the cake circle. Brush layer with rum syrup. Spread half of the walnu