Add the all-purpose flour, brown sugar, granulated sugar, cinnamon if using, and fine sea salt to a medium-sized bowl. Whisk for 20 to 30 seconds so the dry ingredients are evenly combined.
Scatter the cold butter pieces across the top of the flour mixture instead of adding them in one pile.
Use your fingertips to rub the butter into the dry ingredients. Lift and press lightly until the mixture forms uneven crumbs and no longer looks dry.
Use the crumble topping immediately over fruit desserts, muffins, pies, or cobblers, then bake according to the dessert recipe until the topping is golden and crisp.