Line a large sheet pan with parchment or foil and set it aside.

Preheat your oven to 425°F.

In a large bowl, whisk together flour, baking powder, baking soda, sugar, and salt until evenly combined.

Toss the cold butter cubes into the dry mix. Use a pastry cutter or your fingertips to rub the butter into the flour until you have a cornmeal-like texture with pea-sized butter bits.

Make a well in the center and add the cold sour cream. Stir gently until a shaggy dough forms.

If the bottom of the bowl has lots of dry flour, add 1 tablespoon milk and mix.

Turn the dough out onto a lightly floured surface and bring it together into a rough square.

Cut the square into four smaller squares. Stack and press those pieces, then repeat the cut-and-stack process two more times to build layers.

Pat the dough into a rectangle at least 1/2 inch thick. Use a 2-inch cutter to cut biscuits.

Gather scraps gently and cut until you've used the dough. A 2-inch cutter should yield up to 12 biscuits.

Place the cut biscuits on the prepared sheet pan and freeze for 10–15 minutes.

Whisk one egg with 1 tablespoon water and lightly brush the tops of the chilled biscuits.

Bake at 425°F for 18–20 minutes until tops are golden brown.

Cool briefly on a rack, then serve warm.