How to Make It
Pulse baking powder, salt, sugar, baking soda, and 4 cups flour in a food processor until combined. Add cold butter and pulse until mixture is crumbly, with some pea-size pieces of butter remaining.
Transfer mixture to a large bowl and add milk, mixing with a fork until a very shaggy dough forms. Transfer to a lightly floured surface and pat into a 1-inch-thick square. Cut dough into 4 equal pieces, stack pieces, and pat out again into a 1-inch-thick square. Using a sharp knife, cut dough into 16 2-inch squares.
Transfer biscuits to a parchment-lined baking sheet, spacing them ½ inch apart, and freeze for 10 minutes.
Preheat oven to 400°F. Brush tops of biscuits with melted butter and bake until golden, 13 to 15 minutes. Serve warm or at room temperature.
How to freeze and reheat: Arrange unbaked biscuits in a single layer on a parchment-lined baking sheet and freeze until firm. Transfer frozen biscuits to a resealable plastic bag and store in the freezer for up to 1 month. Bake as directed; no need to thaw first.