Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.

In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt.

In a separate bowl, mix the melted unsalted butter, milk, large egg, and vanilla extract until smooth.

Gently combine the wet mixture into the bowl of dry ingredients, mixing just until blended.

In a small bowl, combine the brown sugar, ground cinnamon, and melted butter until it forms a paste.

Spoon the muffin batter into the prepared muffin cups, then add a teaspoon of the cinnamon filling mixture, swirling gently.

Top each muffin cup with the remaining batter, filling three-quarters full.

Bake for 15 to 20 minutes, until a toothpick inserted comes out clean.

Prepare the icing by whisking powdered sugar with a splash of milk and vanilla extract, adjusting the consistency as needed.

Once cooled slightly, drizzle the icing generously over each muffin before serving.