Preheat your oven to 375°F (190°C). Grease a standard 12-cup muffin tin with non-stick spray or line it with paper liners. This ensures your muffins come out cleanly after baking.
In a large mixing bowl, whisk together the all-purpose flour, 1 cup of sugar, baking powder, salt, and 1 1/2 teaspoons of cinnamon. Make sure there are no large clumps so the spice is evenly distributed.
In a separate medium bowl, beat the egg lightly. Whisk in the milk, 1/3 cup of melted butter, and the vanilla extract until the mixture is smooth and well combined.
Create a well in the center of the dry ingredients and pour in the wet mixture. Use a spatula to fold the ingredients together gently. Stop as soon as the flour streaks disappear; the batter should still look a bit lumpy.
Prepare the topping by mixing 1/4 cup sugar and 1 teaspoon cinnamon in a small bowl. Spoon the batter into the prepared muffin tin, filling each cup about three-quarters full. Drizzle the extra 2 tablespoons of melted butter over the tops and sprinkle generously with the cinnamon-sugar mixture.
Place the tin in the center of the oven and bake for 18 to 20 minutes. The muffins are done when a toothpick inserted into the center comes out clean or with just a few moist crumbs. Let them cool in the pan for 5 minutes before transferring to a wire rack.