2 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon salt
1/4 teaspoon baking soda
7 tablespoons unsalted butter, chilled in freezer and cut into thin slices
3/4 cup cold buttermilk
2 tablespoons buttermilk for brushing

Preheat oven to 220 degrees C.
Line a baking sheet with a silicone baking mat or parchment paper.
Whisk flour, baking powder, salt, and baking soda together in a large bowl.
Cut butter into flour mixture with a pastry blender until the mixture resembles coarse crumbs, about 5 minutes.
Make a well in the center of butter and flour mixture. Pour in 3/4 cup buttermilk; stir until just combined.
Turn dough onto a floured work surface, pat together into a rectangle.
Fold the rectangle in thirds. Turn dough a half turn, gather any crumbs, and flatten back into a rectangle. Repeat twice more, folding and pressing dough a total of three times.
Roll dough on a floured surface to about 1/2 inch thick.
Cut out 12 biscuits using a 2 1/2-inch round biscuit cutter.
Transfer biscuits to the prepared baking sheet. Press an indent into the top of each biscuit with your thumb.
Brush the tops of biscuits with 2 tablespoons buttermilk.
Bake in the preheated oven until browned, about 15 minutes.