Mix together flour, sugar, brown sugar, cinnamon, and cubed butter using an electric mixer. Beat at a high speed until big crumbles form. Spread out crumbles on a baking sheet. Put the baking sheet in the oven along with the cake, on a different rack. . Bake for 4 minutes then remove from oven and scrape crumbs back into the bowl. Carefully sprinkle cooked crumbles on top of the cake. When done, remove the coffee cake from the oven and let it cool.