Beat butter until creamy. Beat in sugar until fluffy. Add eggs, one at a time, beating well. Mix in vanilla, baking powder and salt. Add mashed blueberries. Fold in half the flour with spatula, then milk, then remaining flour. Fold in blueberries. Spoon into muffin cups. Combine sugar and nutmeg and sprinkle on top of muffin mixture.
Bake at 375º for 20-25 minutes. Cool in muffin tins. Great to freeze. Just pop in microwave for 25 seconds for a warm snack anytime of day!