1. Preheat your oven to 350°F and prepare a 9×13-inch baking pan by greasing it with butter or lining it with parchment paper.

2. In a large bowl, combine the melted butter and granulated sugar, stirring until well mixed.

3. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until fully incorporated.

4. In a separate bowl, whisk together the mochiko flour and baking powder. Gently fold the dry ingredients into the wet mixture until just combined, being careful not to overmix.

5. Slowly pour in the evaporated milk and coconut milk, mixing until the batter is smooth and free of lumps.

6. If using, fold in the shredded coconut, distributing it evenly throughout the batter.

7. Pour the prepared batter into the greased baking pan, smoothing the top with a spatula.

8. Bake the Hawaiian Butter Mochi for 50-60 minutes, or until the top is lightly golden brown and a toothpick inserted into the center comes out mostly clean, with a few moist crumbs attached.

9. Allow the mochi to cool completely in the pan before cutting into squares or rectangles for serving.