Preheat the oven to 375 degrees F. Grease a muffin tin or line the cups with paper liners. In a mixer fitted with a paddle attachment (or using a hand mixer), cream the butter until smooth. Add 1 cup of the sugar and mix. Add the eggs, vanilla, baking powder, and salt and mix. In a shallow bowl, mash 3/4 cup of the blueberries with the back of a fork. Add to the batter and mix.