Preheat oven to 400 degrees. In a large bowl, whisk together flour, salt, pepper, cayenne pepper, and baking powder. Add cheddar and toss to coat. Add sausage, onion, and butter. With your hands, mix until well combined and roll mixture into 1-inch balls. Place balls, 1/2 inch apart, on a parchment-lined baking sheet. Bake until balls are golden and cooked through, 25 minutes. Serve warm.