Preheat oven to 325°F (170°C). Spray 2 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.

In a large bowl, whisk together brown sugar, buttermilk, mashed banana, granulated sugar, oil, eggs, and vanilla.

In a medium bowl, whisk together flour, baking soda, baking powder, and salt. Slowly add flour mixture to brown sugar mixture, whisking just until combined. Divide batter between prepared pans.

Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks.

Level cakes, if desired. Place 1 cake layer on a serving platter. Place Chocolate Buttercream in a piping bag fitted with a 9/16-inch (about ½-inch) round piping tip (Ateco #807). Starting from pastry edge and working toward center, pipe tall circles (about 1¾ inches at base of circle) on cake layer. Top with remaining cake layer. Pipe another layer of tall buttercream circles. Use any remaining buttercream to fill in any holes and decorate top of cake. Garnish with chopped chocolate, if desired. Refrigerate in an airtight container for up to 5 days.