Preheat oven to 375°F. Grease 12 muffin cups generously; a vegetable oil pan spray works well.
Cream together butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Whisk together flour, baking powder and salt, add to the creamed mixture, beat well. Stir in milk and vanilla (and opt. lemon zest), mixing only until smooth. Quickly but gently fold in the berries.
Fill the muffin cups 3/4 full. Sprinkle with opt. sugar. Bake at 375 for 30 minutes, until light golden brown, remove from the oven, and place the pan on a rack to cool for 10 minutes. After 10 minutes, run a knife around the edge of one muffin, and try to lift it out of the pan. (These muffins are very tender, and if they don't cool long enough, they'll crumble when you try to remove them from the pan.) If the muffin lifts out of the pan without crumbling, remove all of the muffins and cool them completely on a wire rack. If the muffin sticks or the top peels off, allow them to cool an additional 10 minutes, then try again. Yield: 12 muffins.