Preheat & Prep: Preheat oven to 400°F (200°C). Grease a 12‑cup muffin tin or line with paper liners.
Cook Sausage: In a medium skillet over medium heat, crumble and brown sausage until no pink remains, about 6–8 minutes. Drain on paper towels.
Mix Dry Ingredients: In a large bowl, whisk together flour, baking powder, baking soda, salt, and pepper.
Mix Wet Ingredients: In another bowl, whisk eggs, milk, sour cream (or yogurt), and melted butter until smooth.
Combine: Pour wet mixture into dry ingredients; stir gently until just combined (lumpy is okay).
Fold In Add‑Ins: Gently fold in cooled sausage and cheddar until evenly distributed. If using, fold in chives.
Fill & Bake: Divide batter into muffin cups, filling each about ¾ full. Bake 18–22 minutes, until golden and a toothpick comes out clean.
Cool: Let muffins rest in pan 5 minutes, then transfer to a rack to cool slightly before serving.