Preheat oven to 425°F (218°C). Line a 12-count muffin pan with muffin liners. Set aside.

In a small bowl, mix all of the topping ingredients together. Set aside.

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

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar together on medium-high speed until smooth and creamy, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the sour cream, eggs, and vanilla extract, and beat on medium speed until combined. Add in the dry ingredients and beat on low speed until almost combined. Add the milk and beat on low speed until combined. Batter is thick and creamy. Fold in the blueberries.

Spoon the batter into liners, filling them all the way to the top. Spoon brown sugar topping on each, gently pressing it down into the surface so it sticks. Bake for 5 minutes at 425°F and then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C). Bake for an additional 16–19 minutes or until a toothpick inserted in the center comes out clean. Allow the muffins to cool for 5–10 minutes in the pan set on a cooling rack, then transfer the muffins directly to the rack to continue cooling.

Muffins stay fresh covered at room temperature for a few days or in the refrigerator for up to 1 week.