Prepare the Oven and Muffin Pan: Preheat your oven to 375°F. Line a 12-cup muffin pan with paper liners or lightly grease each muffin cup.
Mix the Dry Ingredients: In a large bowl, whisk together the all-purpose flour, baking powder, salt, and ground cinnamon until evenly combined.
Mix the Wet Ingredients: In a separate bowl, whisk together the granulated sugar, melted butter, eggs, milk, and vanilla extract until smooth and well combined.
Combine the Batter: Gradually add the dry ingredients to the wet mixture. Gently fold with a spatula just until combined. Avoid overmixing, as this can make the muffins dense.
Prepare the Cinnamon Filling: In a small bowl, combine the brown sugar, ground cinnamon, and melted butter. Stir until the mixture is evenly combined and fragrant.
Fill the Muffin Cups: Spoon a layer of muffin batter into each prepared muffin cup. Add a small spoonful of the cinnamon mixture to the center, then cover with more muffin batter.
Create the Cinnamon Swirl: Use a toothpick or skewer to gently swirl the cinnamon mixture through the batter. Don't over-swirl so the cinnamon ribbons remain visible.
Bake the Muffins: Bake for 18–22 minutes, or until the muffins are lightly golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool the Muffins: Let the muffins cool in the pan for a few minutes, then transfer them to a wire rack to cool further.
Make the Glaze: Whisk together the powdered sugar, milk, and vanilla extract until smooth. Add a little more milk if needed to reach a drizzleable consistency.
Glaze and Serve: Drizzle the glaze over the cooled muffins. Serve them slightly warm or at room temperature for the best cinnamon roll flavor.