Preheat & Prep: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly coat with nonstick spray.
Mix the Wet Ingredients: In a medium bowl, whisk together pumpkin puree, melted butter, sugar, egg, and vanilla until smooth and glossy.
Combine the Dry Ingredients In another bowl, whisk flour, baking soda, baking powder, salt, and cinnamon.
Fold Together: Add the dry ingredients into the wet mixture and fold gently until just combined. (Don’t overmix — that’s the secret to soft, fluffy muffins.)
Make the Cinnamon Swirl: Stir together the melted butter, brown sugar, and cinnamon in a small bowl until syrupy.
Assemble: Fill each muffin cup halfway with batter. Add a spoonful of cinnamon swirl, then cover with more batter. Use a toothpick or skewer to swirl lightly through each cup.
Bake: Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean and the tops are lightly golden.
Glaze: While the muffins cool slightly, whisk powdered sugar, vanilla, and milk in a small bowl. Drizzle over warm muffins and let set.