In separate bowl, combine flour and baking powder. Add the flour mixture to the butter/sugar/egg mixture, then add 3/4 cup of milk, vanilla, and lemon extract. Carefully fold in frozen blueberries.
Divide the batter into the 6-cup muffin pan. Bake on the middle rack of the oven for 50-60 minutes or until toothpick inserted in center of muffin comes out clean.