Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.

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

In another bowl, mix melted butter, eggs, milk, and vanilla until smooth.

Pour wet ingredients into dry mixture and stir gently until combined.

Fold in sour cream until smooth but do not overmix.

In a small bowl, mix brown sugar and ground cinnamon.

Scoop batter into muffin liners, sprinkle with cinnamon-sugar mixture, and top with more batter.

Bake for 18-20 minutes until golden and a toothpick comes out clean.

Cool muffins in the tin for 5 minutes, then prepare the glaze.

Drizzle the glaze over warm muffins and serve.