Preheat the oven to 350°F Spray a 9-inch square cake pan with Pam.
Make topping: in a small bowl, stir together brown sugar, chocolate chips and cocoa.
Make cake: in a large bowl, beat sugar, oil, coffee, egg, egg white and vanilla.
In another bowl, stir together flour, cocoa, baking powder and baking soda. With a wooden spoon, stir the dry ingredients into the coffee mixture in batches, alternating with the sour cream, making two additions of dry and one of wet, and stirring just until the mixture is combined. Pour it into a prepared pan. Sprinkle with topping.
Place the pan in the centre of the oven and bake for 30 to 35 minutes or until a tester inserted in the centre comes out clean.