Preheat the oven to 425 degrees.
Whisk Together Wet: Add 1 ½ sticks of melted butter to a large bowl along with the sugar, sour cream, milk, whisked eggs, and vanilla extract. Whisk until well combined and there are no clumps of sour cream remaining.
1 ½ sticks butter, 1 cup sugar, ½ cup sour cream, ½ cup milk, 1 Tbsp. vanilla extract, 2 eggs
Add Dry Ingredients: Add the flour, baking powder, and salt to the wet ingredients. Mix until just combined. It’s ok if some small clumps remain.
2 ½ cups flour, 1 Tbsp. baking powder, ½ tsp. salt
Mix Cinnamon Swirl Ingredients: In a small bowl whisk together ⅓ cup brown sugar and 1 ½ teaspoons of cinnamon.
⅓ cup brown sugar, 1 ½ tsp. cinnamon
Fill Muffin Tray: Line a 12-cup muffin tray with cupcake liners and spray each with non-stick cooking spray. Fill each cavity with the muffin batter to ⅓-½ full. Sprinkle ~1 ½ teaspoons of the cinnamon swirl mixture into each cupcake cavity. Pour in the remaining cupcake batter until they are almost completely full and smooth out the tops. Place a toothpick into each one and make a swirling motion.
Finish with Crumb Topping: In a medium sized bowl combine the crumb topping ingredients. Mix together with a fork until a coarse crumb forms. Sprinkle equal amounts of the crumb topping on top of the muffins, pressing it in lightly to the batter so it sticks.
½ cup brown sugar, ⅔ cup flour, 4 Tbsp. butter, 2 tsp. Cinnamon
Bake in Oven: Bake the muffins for 5 minutes at 425 degrees. Reduce the heat to 375 and continue cooking for an additional 15-17 minutes, or until a toothpick comes out clean.
1 cup powdered sugar, 1-2 Tbsp. milk, ½ tsp. vanilla
Finish with Glaze: Whisk together the glaze ingredients in a medium-sized bowl and drizzle over muffins once cooled to room temperature.