Preheat oven to 375°F (190°C). Line or grease a muffin tin.

In a bowl, whisk together flour, baking powder, salt, and cinnamon if using.

In a separate bowl, mix melted butter, sugars, eggs, milk, and vanilla.

Combine wet and dry ingredients until just mixed. Fold in diced nectarines gently.

Spoon batter into muffin cups, filling ¾ full.

Bake 18–20 minutes, or until tops are golden and a toothpick comes out clean.

Cool slightly and enjoy warm or at room temperature.