Heat oven to 425°F. Line a large baking sheet with parchment.
Cook the sausage if needed, then crumble, drain well, and let cool.
Whisk flour, baking powder, baking soda, salt, pepper, garlic powder, and paprika in a big bowl.
Cut in the cold diced butter until the mix looks like coarse crumbs with pea-size bits.
Stir in cheddar, cooled sausage, and scallions until evenly distributed.
Pour in cold buttermilk and gently mix just until a shaggy dough forms.
Turn dough onto a lightly floured surface. Pat to about 3/4 inch, fold over itself three times, then pat to 1 inch thick.
Cut rounds with a 2 1/2-inch cutter, pressing straight down. Place biscuits close together on the sheet.
Bake 13–16 minutes, until tall and deeply golden on top.
Brush hot biscuits with melted butter. Rest 5 minutes and serve warm.