2 cups all-purpose flour
Center a rack in the oven and preheat the oven to 425 degrees F. Line a baking sheet with parchment or a silicone mat. Add the flour, baking powder, salt, baking soda and brown sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix on low until the ingredients are combined and no lumps of sugar remain. With the mixer still on low, drop in the butter and mix until the mixture is pebbly (don't over-mix). You’ll have pea-size pieces, pieces the size of oatmeal flakes and pieces the size of everything in between. Stop the mixer and remove the mixing bowl.
Stir the sour cream and milk together and pour over the dry ingredients. Gently toss and turn the ingredients together with a spatula until you’ve got a nice soft dough. Add the pecans and reach into the bowl with your hands and give the dough a quick, gentle kneading–3 or 4 turns should be just enough to bring everything together.
Lightly dust a work surface with flour and turn out the dough. Dust the top of the dough very lightly with flour and pat the dough out with your hands until it is about 1/2 inch high. Don’t worry if the dough isn’t completely even–a quick, light touch is more important than accuracy.
Use a biscuit cutter or sharp knife to cut out as many biscuits as you can. By hand or with a small spatula, transfer the biscuits to the baking sheet. (The biscuits can be made up to this point and frozen on the baking sheet, then wrapped airtight and kept for up to 2 months. Bake without defrosting–just add a couple more minutes to the oven time.)
Bake the biscuits for 13 to 16 minutes, or until they are tall, puffed and golden brown. Transfer them to a serving basket. Serve right away with soft butter.