America's Test Kitchen; you can substitute 1 1/4 cups regular buttermilk or soured milk for the milk and powdered buttermilk, but the flavor will be less.
Position oven rack in the middle of the oven; preheat to 450°; line a baking sheet with parchment paper.
Pulse the flour, buttermilk powder, sugar, baking powder, salt, and baking soda together in a food processor to combine, about 3 pulses.
Scatter the butter and shortening evenly over the top and continue to pulse until the mixture resembles coarse meal, about 15 pulses.
Transfer the flour mixture to a big bowl; stir in the milk with a rubber spatula until the dough comes together.
Turn the dough out onto a well floured surface; lightly flour your hands and the dough, and knead the dough gently until uniform, about 30 seconds.
Roll the dough into a 10 inch round, about 1 inch thick.
Using a floured 2 1/2 inch biscuit cutter, stamp (don't twist) out 12 biscuits, gently patting the dough scraps back into a uniform 1-inch thick piece as needed.
Arrange biscuits upside down on the prepared baking sheet, spaced about 1 1/2 inches apart.
Bake for 5 minutes; rotate the pan, decrease heat to 400°, and continue to bake until golden brown, 12-15 minutes.
Transfer to a wire rack, let cool for 5 minutes and serve warm.