In separate bowl, 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. Slowly add dry ingredients and milk to the wet ingredients, mixing until well incorporated. Gently fold in the berries.
5 Spoon batter into muffin cups, filling all the way to the top. Spoon brown sugar topping on each, gently pressing down so it sticks.
6 Bake at 425 degrees for 5 minutes and then reduce heat to 350 degrees (keeping muffins in the oven while temp reduces). Bake for an additional 18-20 minutes or until a toothpick inserted into center of muffin comes out clean.
7 Allow to cool in muffin pan for about 5 minutes and then remove to wire rack to continue cooling.