Preparation

• Preheat oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners; line 3 cups of a second muffin pan with 3 liners (or bake in batches)—this recipe makes 15 muffins. Set aside.
• Make the crumb topping: Mix the brown sugar, cinnamon, and melted butter together in a small bowl until combined. Add the flour and use a fork to gently mix until crumbs form. Place crumb topping mixture in the refrigerator while you make the muffin batter.
• Make the muffins: Whisk the flour, baking powder, baking soda, cinnamon, and salt together in a medium bowl. Set aside.
• In a large bowl, beat butter and granulated sugar until smooth and creamy, about 3 minutes. Add eggs and beat until incorporated. Add sour cream and vanilla, and mix until combined.
• With the mixer running on low speed, add the dry ingredients, followed by the milk. Avoid over-mixing.
• Spoon approximately 3 Tablespoons of batter into each of the prepared muffin cups, filling them about ⅔ full. Spoon crumb topping on each, pressing it down gently.
• Bake for 22–24 minutes or until a toothpick inserted in the center comes out clean.
• Allow the muffins to cool for 5 minutes in the muffin pan(s), then transfer to a cooling rack and let them cool for at least 15 minutes before icing.
• Make the icing (if using): Whisk the confectioners’ sugar, 2 Tablespoons milk or cream, and vanilla extract together. Adjust consistency if necessary.
• Drizzle icing over muffins.
• Iced or plain muffins stay fresh covered at room temperature for a few days or in the refrigerator for up to 1 week.