Preheat oven to 350 degrees and lightly grease a 9 inch cake pan.
In a large mixing bowl, beat together the softened butter and sugar.
Next, add in flour, cocoa, sour cream, eggs, baking powder, baking soda and vanilla. Beat until combined.
Stir in the chocolate chips.
Spread batter into the greased cake pan.
In a small bowl, add in the flour, brown sugar, cocoa and cold butter.
Using a pastry blender or fork and knife, cut the butter into the dry ingredient until crumbs form.
Stir in the chocolate chips.
Sprinkle mixture over top of the cake batter.
Bake for 55 - 60 minutes, checking with a toothpick.
Enjoy!