Preheat the oven to 350°F (175°C).
Line two muffin tins with twenty cupcake liners.
In a large bowl, mash 4 of the bananas. Add the butter, sugar, egg, and vanilla and mix well. Add the flour, baking soda, baking powder, and salt and mix until just incorporated.
Add 2 or 3 tablespoons of batter to each lined muffin well; they will be about two-thirds full. Top each with 3 slices from the remaining 2 bananas, if desired.
Bake for 16 to 18 minutes, until the centers of the muffins spring back when touched and no longer appear wet. Be careful to not bake the muffins for too long—you do not want burnt edges.
Let the muffins cool for about 5 minutes before serving.
Leftovers can be stored in an airtight container at room temperature for 1 or 2 days. The banana slices on top will brown, so they are best consumed immediately.