Preheat the oven to 350˚F (177˚C). Spray the inside and the top of a muffin pan with nonstick spray really well. Set aside.
In a medium mixing bowl, whisk together the sweet rice flour and baking powder. Set aside.
In a large mixing bowl, whisk together melted butter, sugar and vanilla extract. Add the beaten eggs and whisk until combined. Pour in the dry ingredients. Using a spatula, stir until incorporated. Pour in the evaporated milk and coconut milk, whisk the mixture until smooth. Smash up half of the banana with a fork into a puree. Add to the batter and whisk until combined.
Fill each muffin tin half full with batter. Cut the rest of the banana into ¼-inch slices. Place 1½ slices of banana in each tin on top of the batter. Fill the muffin tins with more batter until just below the rim. Make sure the banana is well covered. Hold the muffin tray about 2 inches above the counter and drop a few times to help remove any air bubbles. Sprinkles with desired amount of black sesame seeds.
Bake in the oven for about 40 – 50 minutes, or until the top is browned. Transfer to a cooling rack and cool for 20 minutes. Butter mochi is best to serve warm.