Step 1: Preheat the oven

• Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups well.

Step 2: Prepare the streusel topping

• In a small bowl, mix the flour, brown sugar, and cinnamon. Add the cold butter and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs. Set aside.

Step 3: Cream the butter and sugar

• In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 3 minutes.

Step 4: Add eggs and wet ingredients

• Beat in the eggs one at a time, followed by the vanilla extract, sour cream, and milk. Mix until smooth.

Step 5: Combine dry ingredients

• In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Step 6: Fold in the blueberries

• Gently fold the blueberries into the batter, being careful not to crush them.

Step 7: Fill the muffin tin

• Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full. Sprinkle the streusel topping evenly over the batter.

Step 8: Bake the muffins

• Bake for 20–25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Step 9: Cool and serve

• Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.