Step 1

Make the cake batter: Microwave 2 tablespoons unsalted butter in a 12-ounce or larger microwave-safe mug or deep ramekin on HIGH power until melted, 25 to 30 seconds.

Step 2

Add 1/4 cup packed light brown sugar, 1/4 cup milk, and 1/2 teaspoon vanilla extract. Stir with a fork until the sugar is mostly dissolved. Add 1/3 cup plus 1 tablespoon all-purpose flour, 1/2 teaspoon ground cinnamon, 1/4 teaspoon baking powder, and 1/4 teaspoon kosher salt. Stir with the fork until a smooth, loose batter forms with no pockets of unincorporated flour.

Step 3

Make the streusel topping: Microwave 2 tablespoons unsalted butter in a small microwave-safe bowl on HIGH power until melted, 25 to 30 seconds. Add 3 tablespoons all-purpose flour, 2 tablespoons packed light brown sugar, 1/4 teaspoon ground cinnamon, and 1/4 teaspoon kosher salt. Stir with a fork until moistened and crumbly. Scatter the streusel over the cake batter.

Step 4

Microwave on HIGH power until a toothpick inserted into the center comes out mostly clean with just a few moist crumbs stuck to it, 2 to 2 1/2 minutes. (Cooking time will vary greatly depending on the strength of your microwave. Start with 2 minutes and check for doneness before adding more time.)

Step 5

Make the drizzle (optional): Place 2 tablespoons powdered sugar and 1 teaspoon milk in a small bowl and stir until smooth and thick yet pourable. Add more milk to thin it out as needed. Drizzle over the mug cake and serve immediately.