Step 1: In a medium-sized mixing bowl, whisk together the all-purpose flour, granulated sugar, light brown sugar, cinnamon, and salt until no large lumps of brown sugar remain.
Step 2: Add the cold, cubed butter to the dry ingredients. Use a pastry blender, a fork, or your fingertips to work the butter into the flour mixture.
Step 3: Continue mixing until the Streusel topping recipe mixture resembles coarse crumbs with some pea-sized pieces of butter still visible.
Step 4: Use immediately by sprinkling over unbaked muffins or cakes, or store in an airtight container in the refrigerator for up to 3 days or freezer for 3 months.