Preheat oven to 375 degrees F. Grease 12 muffin cups, or use muffin papers.
In a medium bowl, beat butter until creamy. Add sugar and beat until pale and fluffy. Beat in eggs, then vanilla, baking powder, and salt. Stir mashed berries into batter. Fold half the flour into the butter mixture, then half the milk. Add remaining flour and milk until combined. Fold in remaining blueberries and chocolate chips.
In a separate small bowl, combine ground nutmeg and 1 tablespoon sugar. Scoop batter into muffin cups and sprinkle with nutmeg mixture.
Bake 25 to 30 minutes, until golden brown. Set muffins onto a wire rack and let cool for at least 20 minutes before removing from pan. Serve immediately or freeze until ready to eat.