Step 1

Place 4 tablespoons unsalted butter in a medium microwave-safe bowl and microwave in 15-second bursts until completely melted, 30 to 45 seconds total. (Alternatively, melt in a small saucepan over medium-low heat.)

Step 2

Add 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup packed brown sugar, 1/2 teaspoon ground cinnamon if using, and 1/4 teaspoon kosher salt. Stir with a fork until evenly moistened and a variety of crumb sizes form. Do not mix into a paste of uniform consistency. Refrigerate until the butter firms up, at least 10 minutes.

Step 3

This recipe makes enough to top 12 muffins, 1 (9x5-inch) quick bread or loaf cake, or 1 (8- to 9-inch) square or round cake. To use, sprinkle the streusel evenly over the top of the batter before baking, then bake as directed.