Preheat the oven to 350°F (175°C) and grease a 9-inch baking pan.
Cream butter and sugar until light and fluffy.
Add eggs one at a time, mixing well. Stir in vanilla extract.
In a separate bowl, combine flour, baking powder, baking soda, and salt.
Gradually add dry ingredients to the wet mixture, alternating with sour cream.
Pour half the batter into the prepared pan.
Mix streusel ingredients: brown sugar, pecans, cinnamon, melted butter, and flour.
Sprinkle half the streusel over the batter.
Add remaining batter and top with the rest of the streusel.
Bake for 45 minutes or until a toothpick comes out clean. Cool before slicing.