Preheat oven to 350F. Line a 12-cup muffin tin with muffin wrappers. Set aside.

Prepare the cupcakes: In a large bowl whisk mashed banana, melted butter, honey, eggs, vanilla, and buttermilk until smooth. In another large bowl, whisk flour, baking powder, and salt until combined. Add dry ingredients to liquid and gently stir until just combined.

Fill prepared muffin tin cups 3/4 full of batter. Bake cupcakes at 350F 20 minutes or until a toothpick inserted in middle comes out with a few crumbs sticking to it. Cool cupcakes completely on a wire cooling rack.

While cupcakes cool, prepare the buttercream: Place butter and chocolate peanut butter in a medium bowl. With a hand-held electric mixer, beat on medium-high speed until smooth and creamy, about 1-2 minutes. Add cocoa powder, vanilla, and xylitol, beating until combined. Add cream a tablespoon at a time until a smooth and creamy consistency is reached. Refrigerate frosting until ready to use.

Once cupcakes are cooled, frost cupcakes with buttercream. Garnish with sliced banana if desired.