Prepare a 9x13-inch (23x33 cm) pan with butter and parchment, and heat the oven to 350°F (180°C). The parchment should cover the bottom so the sticky baked slab releases more easily.

Distribute the sugar, baking powder, and salt throughout the mochiko with a whisk in a large bowl. The dry mixture should look uniform, without concentrated pockets of baking powder.

Work the eggs, whole milk, coconut milk, melted butter, and vanilla into the dry mixture with the whisk. The combined batter should flow like thick pancake batter, with no visible dry flour.

Position a fine mesh sieve over the prepared pan and pour the batter through it. The sieve catches stubborn flour clumps while the smooth batter spreads across the pan.

Bake for 50 to 60 minutes, using a golden surface and a set center to judge when to remove the pan. The finished top should look browned rather than pale, and the middle should no longer move like liquid.

Leave the mochi in the pan until it has cooled completely. The center firms as it cools and should feel settled rather than soft and hot.

Divide the cooled slab into 24 squares with a sharp knife. Wipe away sticky residue between cuts to keep the edges neat.