Preheat the oven to 375 degrees Fahrenheit. Line a 12-cup muffin tin with paper liners or grease lightly. In a medium bowl, whisk together flour, sugar, baking powder, and salt. In a separate bowl, combine melted butter, milk, eggs, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir gently until just combined. Avoid overmixing to maintain a tender crumb. The batter should be thick but pourable, similar to traditional muffin consistency.

In a small bowl, mix together brown sugar, granulated sugar, and cinnamon. Stir in melted butter until the mixture is crumbly yet holds together slightly. This cinnamon-sugar swirl provides flavor contrast and visual appeal in each muffin.

Spoon half of the batter into the prepared muffin tin, filling each cup about one-third full. Sprinkle a generous amount of the cinnamon-sugar mixture evenly over the batter. Top with the remaining batter, then finish with the remaining cinnamon-sugar. For a more pronounced swirl, gently swirl a knife through each muffin to create marbled patterns.

Place the muffin tin in the preheated oven and bake for 18 to 22 minutes or until a toothpick inserted in the center comes out clean. Baking times may vary depending on oven and muffin size. Remove muffins from the oven and allow to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

If using the optional glaze, whisk together powdered sugar, milk, and vanilla until smooth. Drizzle over the cooled muffins for added sweetness and visual appeal. The glaze sets quickly, forming a delicate finish that enhances both flavor and presentation.