Place the sugars, cinnamon and salt in a medium bowl.
Pour the melted butter over the sugar mixture and stir until the sugars begin to dissolve.
Stir in the flours all at once with the butter and sugar mixture until all of the ingredients are combined.
Let the streusel sit for 10 minutes, and then, using your fingers, break it up into a pea-sized crumble.
Note: This recipe makes enough streusel to sprinkle another layer on the top of the cake before baking. I like just the one layer so I either make a half recipe or I freeze what is left over).