Preheat Oven: Set your oven to 425°F (220°C). Line a baking sheet with parchment paper or lightly grease it.
Mix Dry Ingredients: In a large bowl, whisk together the whole wheat flour, all-purpose flour, baking soda, baking powder, salt, and sugar.
Cut in Butter: Add the cold butter to the dry ingredients. Use a pastry cutter, cheese grater, fork, or your hands to work the butter into the flour until the mixture looks like coarse crumbs with pea-sized bits of butter throughout.
Add Wet Ingredients: Stir in the sourdough discard and milk into the flour mixture. Mix gently until just combined. Be careful not to overmix, or the biscuits will be tough. The biscuit dough should be slightly sticky but still hold together.
Form Biscuits: Turn the dough out onto a lightly floured surface. Gently pat it into a rough rectangle, about 1-inch thick. Fold the dough over itself a couple of times (this helps create layers), then press it back down to 1-inch thick. Use a round biscuit cutter or a drinking glass to cut out biscuits. For overnight sourdough biscuits that are long fermented, cover and place the biscuits in the fridge overnight and bake the next morning. Otherwise proceed to the next step.
Bake: Place cut biscuits on the parchment-lined baking sheet, leaving a little space between each. Bake biscuits in a 350 degree preheated oven for 10-12 minutes or until golden brown on top.
Serve: Let the biscuits cool slightly on a wire rack, then serve warm with butter, jam, or gravy.