Preheat the oven to 350F (180C) and line a muffin tin with muffin papers. This recipe will make about 14-16 cupcakes, so you will either need to use two muffin tins or bake 1 tin of muffins, let the pan cool and then bake the additional 2-4 cupcakes.
In a medium bowl sift together the flour, baking powder, baking soda and salt. Whisk together. Set aside.
In a large bowl, beat together the butter and sugar until creamed.
Add in the vanilla extract, then beat in the eggs 1 at a time until incorporated (you should no longer see pieces of egg).
Add about 1/3 of the flour mixture into the butter mixture and slowly beat together on a low speed until incorporated. Then stir in the sour cream.
Beat in another about 1/3 of flour mixture into the batter with the mixer on a low speed, then beat in the milk (again with the mixer on a low speed). Then beat in the rest of the flour mixture.
Carefully fold in the chopped berries.
Spoon the batter into the prepared pans, filling each muffin cavity about 2/3 to 3/4 full. You should end up with about 14-16 cupcakes in total. If you only get 12 cupcakes (and you use all of the batter) then your cupcakes are too full and will overflow.
Bake 1 tray at a time in the middle of the preheated oven. They should take about 21-24 minutes, or until the tops look set and an inserted toothpick comes out clean or with a few moist muffin crumbs but no gooey muffin batter.
Cool the muffins in the pan for at least 10 minutes, then carefully transfer to a cooling rack to continue cooling.