Preheat your oven to 350°F (175°C).

Line your muffin tin with paper liners or grease it with non-stick spray.

In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In another bowl, beat the softened butter until creamy, then add granulated sugar and mix until fluffy.

Add the eggs one at a time, mixing well after each addition, then stir in the sour cream and vanilla extract.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

In a small bowl, combine brown sugar, flour, cinnamon, and nuts to prepare the streusel topping.

Scoop the batter into the prepared muffin tin, filling each cup about two-thirds full, and sprinkle the streusel topping over each muffin.

Bake for 20 minutes or until a toothpick inserted comes out clean.

Let the muffins cool in the pan for a few minutes before transferring to a cooling rack.