In a large bowl, whisk the oat, millet, tapioca, cornstarch, baking powder, salt, and pepper. Add the butter pieces and work with your fingertips or pastry blender until butter is size of peas. Chill the mixture for 20 minutes.
Position rack in upper third of oven and preheat to 425 degrees F. Stacked a rimmed baking sheet atop a second rimmed baking sheet (prevents over-browning of bottoms) and line the top on with parchment.
Remove chilled flour mixture and add ricotta and cream. Stir and gently knead with your hands until dough comes together in a rough ball. Dough should feel fairly firm, but evenly moistened. If you need to, you can add 1/2 tablespoon of cream, but just know it doesn't need to be real wet, just stick together.
Turn the dough out onto a light dusted surface with oat flour and form into a disk, keeping the height of the biscuit in mind, so don't make it too flat. Use a biscuit cutter to make into squares, reshaping the disk to cut more squares. You can also cut the disk into squares with a knife, but a biscuit cutter does do something to help the biscuits rise straight and tall.
Freeze the biscuits for 10 minutes on the sheet pan, then brush with a little extra cream
Bake on the upper rack of the oven until golden on top, 15-20 minutes. Do not overbake, because they will continue baking after removed from oven. Let cool for 15 minutes.