Preheat your oven to 350°F (175°C). Grease or line a muffin tin with paper liners to prevent sticking.

In a large mixing bowl, whisk together the sifted all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt. This ensures even distribution of the leavening agents and spices.

In another bowl, combine the canned pumpkin puree, granulated sugar, brown sugar, milk, vegetable oil, and eggs. Whisk until smooth and well combined.

Gradually add the dry ingredients to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix, as this can lead to dense muffins.

In a small bowl, mix together the packed brown sugar, ground cinnamon, and melted butter until it forms a paste. This will create the delicious swirl in the muffins.

Spoon about 2 tablespoons of muffin batter into each muffin cup, filling them about halfway.

Add about 1 teaspoon of the cinnamon swirl mixture on top of the batter in each cup, then cover with another tablespoon of muffin batter. Swirl the batter and cinnamon mixture together gently with a toothpick or knife to create a marbled effect.

Repeat until all muffin cups are filled.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean. This ensures that the muffins are cooked through.

Once baked, remove the muffins from the oven and let them cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

In a small bowl, whisk together the sifted powdered sugar, milk, and vanilla extract until smooth. Adjust the consistency by adding more milk if needed.

Drizzle the glaze over the cooled muffins before serving for a sweet finish.