Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease it well.

In a large mixing bowl, whisk together the flour, 1/2 cup of the granulated sugar, baking powder, salt, and 1 teaspoon of cinnamon. Make sure these dry ingredients are thoroughly combined.

In a separate medium bowl, whisk together the melted butter, egg, milk, and vanilla extract until well blended.

Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are perfectly fine.

Prepare the cinnamon-sugar topping: In a small bowl, combine the softened butter, brown sugar, and the remaining 1/2 cup of granulated sugar, and 1 teaspoon of cinnamon. Use a fork or your fingers to mix until it resembles coarse crumbs.

Spoon the muffin batter evenly into the prepared muffin cups, filling each about two-thirds full.

Generously sprinkle the cinnamon-sugar topping over the batter in each muffin cup.

Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.