Preheat the Oven:

Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or spray with non-stick cooking spray.

Mix the Dry Ingredients:

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

Prepare the Wet Ingredients:

In a large mixing bowl, whisk together the melted butter and sugar until smooth.

Add the eggs and vanilla extract, mixing until fully combined.

Gradually whisk in the milk.

Combine the Batter:

Add the dry ingredients to the wet ingredients, gently folding with a spatula until just combined. Be careful not to overmix.

Make the Cinnamon Sugar Swirl:

In a small bowl, mix the sugar and cinnamon for the swirl.

Assemble the Muffins:

Fill each muffin cup halfway with batter. Sprinkle a generous teaspoon of the cinnamon sugar mixture over the batter, then add another spoonful of batter to cover. Use a toothpick or skewer to swirl the cinnamon sugar into the batter.

Add the Topping:

In a small bowl, mix the sugar and cinnamon for the topping. Sprinkle over the top of each muffin.

Bake:

Bake in the preheated oven for 18–20 minutes, or until a toothpick inserted into the center comes out clean.

Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.