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.

Add ¼ cup packed light brown sugar, ¼ cup milk, and ½ teaspoon vanilla extract. Stir with a fork until the sugar is mostly dissolved. Add ⅓ cup plus 1 tablespoon all-purpose flour, ½ teaspoon ground cinnamon, ¼ teaspoon baking powder, and ¼ teaspoon kosher salt. Stir with the fork until a smooth, loose batter forms with no pockets of unincorporated flour.

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, ¼ teaspoon ground cinnamon, and ¼ teaspoon kosher salt. Stir with a fork until moistened and crumbly. Scatter the streusel over the cake batter.

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 ½ 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.)

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.