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

In a small bowl, combine 1/2 cup flour, 1/3 cup brown sugar, 1 1/2 teaspoons cinnamon, and a pinch of salt for the streusel. Mix well to ensure everything is combined.

Add 1/4 cup melted butter to the streusel mixture and stir until it becomes crumbly. Set it aside.

In a separate bowl, whisk together the 2 cups flour, baking powder, baking soda, and salt.

In a large mixing bowl, cream together the softened butter and 3/4 cup granulated sugar until light and fluffy (about 3-4 minutes).

Add the eggs one at a time, beating well after each addition, then mix in the sour cream and pure vanilla extract until fully blended.

Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix; a few lumps are okay!

Fill each muffin cup with about a tablespoon of batter. Sprinkle a layer of streusel over the batter, then add more batter to cover, and finish with a generous top layer of streusel. For an extra touch, swirl the batter gently with a toothpick.

Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean. While they bake, marvel at that incredible cinnamon smell filling your kitchen!

Once baked, cool them in the pan for about 5 minutes before transferring to a wire rack to cool completely.