Step 1: Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or use cupcake liners. Grease/line a second pan with 2 liners because this recipe yields about 14 muffins. Set aside.
Step 2: Mix all of the topping ingredients together. Set aside.
Step 3: Whisk the flour, baking soda, baking powder, and salt together in a large bowl. Set aside.
Step 4: Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter, granulated sugar, and brown sugar together on high speed until smooth and creamy, about 2 minutes. On medium speed, add the eggs one at a time, beating well after each addition. Beat in the sour cream and vanilla extract on medium speed until combined. With the mixer running on low speed, add the dry ingredients and milk into the wet ingredients and beat until no flour pockets remain. Fold in the blueberries.
Step 5: Spoon the batter into liners, filling them all the way to the top. Bake for 5 minutes at 425 then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C). Bake for an additional 18–20 minutes or until a toothpick inserted in the center comes out clean. The total time these muffins take in the oven is about 23–25 minutes, give or take. Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a wire rack to continue cooling.
Step 6: Muffins stay fresh covered at room temperature for a few days or in the refrigerator for up to 1 week.
I hope your muffins turned out great!