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

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

In a large bowl, cream together the butter and sugar until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, beating well after each addition.

Stir in the vanilla extract.

Gradually add the flour mixture alternating with the milk, beginning and ending with the flour mixture. Mix just until combined.

In a small bowl, mix together the brown sugar, cinnamon, and melted butter until combined.

Spoon about 2 tablespoons of batter into each muffin cup.

Add a teaspoon of the cinnamon mixture on top, then cover with remaining batter.

Take a toothpick or knife and swirl the batter to create a marbled effect.

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

Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack.

While the muffins are cooling, whisk together the powdered sugar, milk, and vanilla extract until smooth.

Once muffins are slightly cooled, drizzle the glaze over the top. Allow glaze to set before serving.