Prepare the baking pan: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal and cleanup.
Mix the dry ingredients: Combine the glutinous rice flour, sugar, and baking powder in a large mixing bowl. Whisk together until well combined, and there are no lumps.
Combine: Combine the wet ingredients. In another bowl, whisk together the melted butter, whole milk, coconut milk, vanilla extract, eggs, and salt. Mix until smooth and well incorporated.
Mix: Mix everything. Gradually pour the wet ingredients into the bowl with the dry ingredients. Stir gently with a spoon or spatula until a smooth batter forms. Ensure there are no dry pockets left.
Pour: Pour the batter into the prepared baking pan, spreading it out evenly with a spatula. Tap the pan gently on the counter to remove any air bubbles. Add coconut flakes (optional): Sprinkle coconut flakes on top for added texture.
Bake: Place the baking pan in the preheated oven and bake for 1 hour or until the top is golden brown, the edges are slightly crispy, and a toothpick comes out clean. The center should be set but still slightly jiggly.
Cool and slice: Let the mochi cool in the pan for 10 minutes before transferring it to a wire rack.
Serve and enjoy: Cut the cooled mochi into squares using a plastic knife for a clean cut. Serve at room temperature or chilled. Optionally, you can sprinkle additional coconut flakes on top of Hawaiian butter mochi for extra texture and flavor!