Preheat oven to 375°F (190°C). Line a 12‑cup muffin tin with paper liners or grease the cups.

In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon.

In a separate bowl, whisk melted butter, sour cream, eggs, and vanilla until smooth and combined.

Pour the wet ingredients into the dry ingredients and stir until just combined—don’t overmix; some lumps are okay.

In a small bowl, mix together the extra sour cream, sugar, and cinnamon until creamy.

Divide half the muffin batter evenly among the 12 cups.

Drop half of the cinnamon‑sugar cream by spoonfuls into each cup, then top with the remaining batter.

Use a toothpick or skewer to swirl the cream into the batter, creating a marbled effect.

Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.