Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease lightly.

Make the streusel: In a medium bowl, combine 1/2 cup brown sugar, 1/2 cup all-purpose flour, and 1 teaspoon ground cinnamon. Add cold, cubed butter and use a fork or your fingers to mix until crumbly. Set aside.

In a large bowl, whisk together 1 1/2 cups all-purpose flour, baking powder, baking soda, and salt.

In another bowl, whisk granulated sugar, melted butter, eggs, sour cream, and vanilla extract until smooth.

Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.

Spoon the batter into the prepared muffin cups, filling each about ¾ full. Sprinkle the streusel generously on top of each muffin.

Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean. If the streusel browns too quickly, cover loosely with aluminum foil during the last few minutes of baking.

Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

Optional: Prepare a simple glaze by whisking together powdered sugar, milk, and vanilla extract until smooth. Drizzle over the cooled muffins.

Serve and enjoy!