Keep these biscuits in the oven a couple minutes longer and you’ve got dryer, crisper results. To serve a crowd or make extra to freeze, double the recipe, forming the dough into 2 equal rounds for 12 biscuits. Another make-ahead tip: mix the flours and butter early in the day and store them in the fridge in an airtight container until you’re ready to bake; it takes mere minutes to mix in the ricotta and cream, then shape, cut and bake the biscuits. Be sure to buy gluten-free oat flour; many brands are not GF. The biscuits are best the day they’re made, but keep at room temperature for a day or two. Toast leftover biscuits before serving.
In a large bowl, whisk together the oat, millet, and tapioca flours with the cornstarch, sugar, baking powder, and salt. Add the butter pieces, and work with a pastry blender or your fingertips until the butter is broken down into the size of small peas. Chill the mixture until cold, 20 to 30 minutes.
Position a rack in the upper third of the oven and preheat it to 425ºF.
Stack a rimmed baking sheet atop a second rimmed baking sheet and line it with parchment paper (this keeps the bottoms from over-browning).
Remove the flour mixture from the refrigerator. Add the ricotta, 3 tablespoons (45 milliliters) of cream, and the chives. Stir and/or knead with your hands until the dough comes together in a rough ball. The dough should feel fairly firm, but evenly moistened.
Lightly dust a clean work surface with oat flour.
Working quickly to keep the dough cold, turn the dough out onto the prepared work surface and form it into a 1-1/4-inch thick disk. If the dough is crumbly, lightly press it together until it comes together. Cut the disk into 6 wedges and place them on the prepared baking sheet.
Brush the top of each biscuit with the remaining 2 teaspoons (10 milliliters) cream and top with a sprinkle of pepper. (Optionally, for taller biscuits: chill the biscuits until firm, 30 to 60 minutes.)
Bake the biscuits on the upper rack of the oven until golden on top, 15 to 20 minutes. Let cool for at least 15 minutes before enjoying; they’re still cooking from residual heat.