Preheat the oven to 375°F (190°C) and line a muffin tin with paper liners or grease it well.
In a large bowl, whisk together the flour, sugar, baking powder, salt, and 1 tablespoon of cinnamon.
In a separate bowl, combine melted butter, milk, and the egg until well mixed.
Pour the wet ingredients into the dry ingredients and stir just until combined—do not overmix.
In a small bowl, mix the remaining tablespoon of cinnamon with a teaspoon of melted butter.
Spoon half the batter into each muffin cup, then add a small spoonful of the cinnamon butter mixture on top, swirling gently with a toothpick or knife.
Top with the remaining batter, and lightly swirl the rest of the cinnamon butter mixture on top of each muffin.
Bake for 18–20 minutes or until a toothpick inserted into the center comes out clean.
Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely or enjoy warm.