Combine the flour, baking powder, sugar, cornstarch, baking soda, and salt.
Grate the frozen butter, then stir it into the flour mixture.
Mix the buttermilk into the flour mixture until it forms a dough.
Sprinkle a work surface with a light coating of flour. Press the dough down onto the floured surface, then fold it 4 to 5 times.
Form the dough into an 8x10-inch rectangle, about ¾-inch thick.
Using a biscuit or cookie cutter, cut the dough into 2 ½-inch rounds. Re-form the remaining dough into another rectangle and continue cutting until you've used all of it up.
Line a baking sheet with parchment paper and arrange the biscuits so that they are just touching.
Brush the tops of the biscuits with the cream, and sprinkle the biscuits with flaky sea salt if desired.
Bake the biscuits for 12 to 15 minutes, until they appear golden.
Let the biscuits cool slightly before you eat them.