Preheat the oven to 350°F (175°C). Grease and flour the Bundt pan.
Combine flour, baking powder, and baking soda in a bowl.
Cream butter and sugar until fluffy.
Add eggs one at a time, mixing well after each. Stir in vanilla.
Gradually add the dry ingredients, alternating with sour cream.
For the streusel, mix brown sugar, cinnamon, flour, pecans, and melted butter until crumbly.
Pour half of the batter into the pan. Sprinkle half the streusel.
Add the rest of the batter and top with remaining streusel.
Bake for 50 minutes or until a toothpick inserted comes out clean.
Cool for 10 minutes in the pan, then transfer to a cooling rack.