Cinnamon Blueberry Muffins

3/4 stick (6 tablespoons) unsalted butter, melted and cooled
1 cup packed light brown sugar
1/2 cup whole milk
1 large egg
1 1/2 cups all-purpose flour
1 1/2 teaspoons baking powder
1 teaspoon cinnamon
1/2 teaspoon salt

1 1/2 cups blueberries (7 1/2 oz)

Put oven rack in middle position and preheat oven to 400°F. Put liners in muffin cups.

Whisk together butter, brown sugar, milk, and egg in a bowl until combined well. Whisk together flour, baking powder, cinnamon, and salt in a large bowl. Add milk mixture and stir until just combined. Fold in blueberries gently. Divide batter among muffin cups and bake until golden brown and a wooden pick inserted into center of a muffin comes out clean, 25 to 30 minutes.