Line an 8-inch square baking pan with parchment paper, letting it overhang on two sides. Spray generously with oil or brush with neutral oil.
In a large mixing bowl, whisk together the mochiko, sugar, and salt until no lumps remain.
In a separate bowl, whisk the milk, water, melted butter, vanilla extract, and black sesame paste until the paste is fully dissolved and the mixture is smooth.
Pour the wet ingredients into the dry ingredients and whisk vigorously until you have a completely smooth, pourable batter with no flour pockets.
Pour the batter into the prepared pan and tap it firmly on the counter 3-4 times to release air bubbles.
Set up a steamer with boiling water. Place the pan in the steamer, cover with a clean kitchen towel under the lid to catch condensation, and steam for 35-40 minutes.
Test doneness by inserting a toothpick into the center — it should come out mostly clean with just a few sticky bits. The surface should look set and glossy.
Remove from the steamer and let cool completely in the pan for at least 30 minutes before handling.
Oil a sharp knife generously and slice into 16 squares, wiping and re-oiling the blade between cuts. Dust with potato starch if desired to prevent sticking.
