Brown the butter: In a saucepan, melt the butter over medium heat until it foams. When foaming, add the dry milk powder (if using). Keep cooking over medium heat, stirring, until the melted butter just turns a light amber brown, then turn off the heat (it will keep darkening from residual heat). Set aside to cool slightly.
Heat the oven to 350F. Grease and line a 9×13" metal baking pan.
In a large bowl, whisk together the mochiko, cornmeal, sugar, salt, and baking powder.
Add the eggs, sour cream, and evaporated milk, whisking to combine.
Whisk in the browned butter (it should have cooled slightly). Mix the batter well and pour into the prepared pan.
Bake 35 to 40 minutes, until a toothpick inserted in the center comes out clean.
The 1/2 cup sour cream makes it more moist and bouncy; leave it out for a more typical cornbread texture. Store leftovers airtight at room temperature up to a week. From Kristen Morita on Instagram.
