Preheat the oven to 425°F and line a baking sheet with parchment paper.
In a bowl, whisk together flour, baking powder, baking soda, sugar, and salt.
Using a large-holed box grater, grate the very cold butter into the dry ingredients. (Coat the stick of butter in flour before grating to avoid slippery butter while grating.) Toss the butter into the dry ingredients to combine.
Pour in the buttermilk and mix just until a shaggy dough forms. (Do NOT overmix the dough, you should have a messy dough.)
Transfer the dough onto a floured surface and gently knead until combined. (Do not overwork the dough, you do not want smooth dough, it's okay if there are some dry spots, and wet spots.)
With your hands or a rolling pin, flatten the dough into a rectangle, about 3/4 inches thick.
Fold one side of the rectangle dough into the center, then top with the other side. Turn the dough and roll the dough out again into a 3/4-inch-thick rectangle and repeat the folding again.
Do the folding method no more than 2-3 times. Finally, after the last fold, roll out the dough into a rectangle again, 3/4 inches thick.
Using a biscuit cutter, press straight down into the dough to cut out round biscuits. Make your cuts close together to get as many biscuits as possible. (Avoid twisting the cutter, spinning it can seal the edges and prevent the layers from rising.)
Gather the leftover biscuits dough and gently press it together and pat it out to be ¾ inches thick. Cut out as many additional biscuits as you can.
Gently add the biscuits onto the lined baking sheet, spacing them slightly apart. Brush the tops with egg wash or milk. (Brushing them helps them brown up, but this is optional.)
Bake at 425°F for 18-20 minutes, until the biscuits are puffed and the tops are golden.
*If you are using store-bought premade biscuits, follow the package instructions to bake.
While the biscuits are baking prepare the sausage gravy.