Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan and line it with parchment paper.

In a mixing bowl, combine melted unsalted butter with granulated sugar until well incorporated. Add evaporated milk, coconut milk, and vanilla. Mix in eggs until smooth.

In another bowl, whisk together glutinous rice flour and baking powder. Gradually mix dry ingredients into the wet batter until just combined.

Pour the batter into the prepared baking pan and smooth the top. Bake for about 60 minutes or until golden brown and a toothpick comes out clean.

Allow the butter mochi to cool in the pan for 15-20 minutes. Remove from pan and let it rest on a wire rack until fully cool before slicing.