Preheat the oven to 350°F (175°C) and grease a 9-inch cake pan.
In a bowl, combine flour, baking powder, baking soda, and salt.
Cream butter and sugar until light and fluffy.
Beat in eggs one at a time, mixing well after each addition.
Add sour cream and vanilla extract.
Gradually fold in dry ingredients.
Mix in chocolate chips.
Pour the batter into the cake pan.
For the streusel, combine flour, cocoa powder, brown sugar, and melted butter. Mix until crumbly.
Sprinkle streusel topping evenly over the cake batter.
Bake for 40 minutes or until a toothpick inserted in the center comes out clean.
Cool before slicing.