Stir 1 cup all-purpose flour and ⅓ cup granulated sugar together in a large bowl.
Cut ⅔ stick butterinto small pieces and add it.
Rub the butter into the dry ingredients with your fingertips or a pastry blender, working quickly, until the mixture forms uneven, pea-sized crumbs. Stop before it comes together into a dough.
For the crispest topping, chill the crumbs a few minutes to firm up, then scatter them over your cake just before baking.
Notes
Makes enough for a 9×13-inch (23×33 cm) pan.
Traditional ratio is about 2 parts flour to 1 part each sugar and butter — as much flour as the sugar and butter weigh together.
The butter should be cool and only slightly softened, never melted, or the crumbs turn to paste.
If the crumbs feel greasy or melt in the oven, work in a spoonful each of flour and sugar.
For a topping that stays crunchy the next day, mix in an egg yolk plus a little extra flour and sugar.
Stores airtight in the refrigerator about a week; freezes well spread in a thin layer with the crumbs kept loose.