Preheat the oven to 350°F and lightly grease an 8x8" baking pan with the wrapper of the butter. Melt the butter.
In a large mixing bowl, whisk together eggs, milk, black sesame paste.
Whisk in glutinous rice flour, baking powder and salt. Mix until everything is very well combined. Don't worry about overmixing because mochiko does not have gluten.
Pour in the melted butter and mix well.
Pour the batter into the pan. Tap the baking pan against the countertop to level out the batter and pop any air bubbles.
Evenly sprinkle sesame seeds in a thin layer over the top of the batter if you are using. Bake the Black Sesame Mochi Cake for about 1 hour, or until it no longer wiggles in the center when you shake the pan. You won't be able to tell by color because the cake is already dark.
Place the pan on a wire rack and run a knife with a thin blade around the edge of the pan to loosen the sides. Let the Black Sesame Mochi Cake cool in the pan completely, about 1 hour.
Carefully, so you don't dislodge too many sesame seeds from the top, tip the Black Sesame Mochi Cake out on to a cutting board upside down. Slice the finished cake from the bottom side with a plastic knife for the cleanest cuts, and serve or wrap for storage.