1Preheat the oven to 375F. In a large bowl, beat the butter and sugar (with the lemon zest, if you're using it) until pale and fluffy. Beat in the eggs and vanilla.
2Add half the flour along with the baking powder and salt and stir just until combined; stir in the milk, then the remaining flour along with the berries, still frozen if they began that way.
3Divide the thick batter between 12 paper-lined muffin cups, filling them full; if you like, sprinkle the tops with sugar. Bake for 20-25 minutes, or until golden and springy to the touch. Serve warm.