Preheat the oven to 350°F (175°C) and grease the muffin tin.

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

In another bowl, mix the melted butter, sour cream, egg, and vanilla extract until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined.

In a small bowl, combine the flour, brown sugar, cold butter, and cinnamon.

Use a fork or your fingers to mix until crumbly.

Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the streusel topping evenly over each muffin.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool for a few minutes before transferring to a wire rack.