In a separate mixing bowl, add topping ingredients (brown sugar, oats, flour, 1 tsp cinnamon, salt, and diced cold butter). Use a pastry cutter, or fork, to cut the butter into the oat mixture, using a slight downward twisting motion, until mixture resembled pea-sized crumbs. Alternatively, you can use two forks, or even your hands, to cut butter into the mixture.