Preheat oven to 425 degrees F. Line a baking sheet with parchment paper; set aside.
In a large bowl, whisk together the flour, baking powder, baking soda and salt. Add the butter and, using a pastry cutter or your fingers, cut the butter into the dry ingredients until the mixture resembles small crumbs the size of peas. Stir in the sausage and cheese.
Add the buttermilk and use a fork to gently stir the mixture until all of the dry ingredients are moistened. Use your hands to knead the mixture a few times in the bowl to bring the dough together.
Turn the dough out onto the parchment-lined baking sheet and pat into an 8x10-inch rectangle. Cut the dough into 12 squares and separate them on the sheet so they are at least 1 inch apart. Brush the tops of each biscuit with the melted butter.
Bake until the biscuits are golden brown, about 20 to 25 minutes. Remove the biscuits from the oven and allow to cool slightly before serving. While these are best the day they are made, any leftovers can be stored in an airtight container, or wrapped in plastic wrap, and stored in the refrigerator for up to 3 days. Re-warm in the microwave or oven.
Both the sausage and cheese amounts can be cut in half if you'd prefer less "filling" in your biscuits.
Sour cream or plain yogurt can be substituted for the buttermilk.
Feel free to play around different types of meats and cheeses, i.e. bacon, ham, swiss, Gruyere, etc. in place of the sausage and cheddar.