Preheat oven to 375°F. Prepare the muffin tins. Grease the sides and bottoms with butter and dust with flour, tapping out any excess. Sift together the flour, baking powder, and salt. Set aside. Place the softened butter and the sugar in a mixing bowl and blend on high speed until the mixture becomes light in color and fluffy. Add the eggs one at a time, mixing until each egg is incorporated. Scrape the bowl occasionally. Mix in the vanilla. Using a rubber scraper, mix in half of the flour and half of the milk. Repeat until all the flour and milk are added. Gently stir in the berries. Scoop into 12 equal muffin cups. Bake for about 18-20 minutes. Remove from the tins to a wire rack and allow to cool. Makes 1 dozen.