Preheat oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners; line 3 cups of a second muffin pan with 3 liners (or bake in batches)—this recipe makes 15 muffins. Set aside.

Mix the brown sugar, cinnamon, and melted butter together in a small bowl until combined. Add the flour and use a fork to gently mix until crumbs form. Don’t over-mix into a paste. Just mix until it is crumbly. It will seem like a lot of crumb topping, and it is! These are generously topped. Place crumb topping mixture in the refrigerator while you make the muffin batter.

Whisk the flour, baking powder, baking soda, cinnamon, and salt together in a medium bowl. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on high speed until smooth and creamy, about 3 minutes. Scrape down the sides of the bowl as needed. Add the eggs and beat on medium speed until incorporated. Add the sour cream and vanilla, and beat until the mixture is combined and mostly creamy. Scrape down the sides of the bowl as needed.

With the mixer running on low speed, add the dry ingredients, followed by the milk. Avoid over-mixing—you may need to switch to stirring with a spatula or whisking by hand to be sure no flour pockets are hiding at the bottom of the bowl. Batter is thick.

Spoon approximately 3 Tablespoons of batter into each of the 15 prepared muffin cups, filling them about ⅔ full. Spoon crumb topping on each, gently pressing it down so it adheres to the batter.

Bake for 22–24 minutes or until a toothpick inserted in the center comes out clean. It’s normal for some of the crumb topping to fall off the sides as the muffins expand, but most should remain securely on top.

Allow the muffins to cool for 5 minutes in the muffin pan(s), then transfer to a cooling rack to continue cooling. Let them cool for at least 15 minutes before icing.

Whisk the confectioners’ sugar, 2 Tablespoons milk or heavy cream, and vanilla extract together. Add another Tablespoon of milk or heavy cream to thin out if necessary. For thicker icing, add a little more confectioners’ sugar. Drizzle over muffins.