In a medium mixing bowl, combine the flour, sugars, cinnamon, and salt
Pour in the melted, cooled butter and stir with a fork until large clumps form and no dry flour remains.
Refrigerate the mixture until ready to use.
Notes
Allow the butter to cool after melting. If the butter is too warm, it will dissolve the sugar, and the mixture will resemble a paste.
Use all brown sugar for more richness. While I prefer using both white sugar and brown sugar, using only brown sugar will give you a richer streusel with a stronger caramel-like flavor.
Uneven clumps are ideal. A streusel topping is “messy” by nature, so don’t worry about trying to make it neat and uniform. While you mix everything up with a fork, clumps of different sizes will form, and that’s precisely what you’re looking for.