Combine the granulated sugar, light brown sugar, kosher salt, and cubed butter in the bowl of a stand mixer fitted with the paddle attachment. Add the flour and mix on medium low until the mixture comes together in large, shaggy clumps with no visible dry flour remaining.

Alternatively, combine all ingredients in a large bowl and squeeze everything together with your hands until the mixture clumps and no loose flour remains.

Turn the mixture out onto a parchment-lined baking sheet and crumble it into clusters of varied sizes. Refrigerate for 20 to 30 minutes.

To bake the streusel on its own, preheat the oven to 350°F conventional while the streusel chills. Bake for 8 to 12 minutes, until the crumbles are golden brown and the centers of the larger clusters look matte rather than shiny. Bake time will vary depending on crumble size.

To use unbaked, crumble the chilled streusel directly over the top of unbaked muffins, quick breads, cakes, or pies and bake as directed by the base recipe. Streusel can add several minutes to the total bake time if the original recipe did not include it.