1 1/2 tablespoons unsalted butter
3 tablespoons whole or 2% milk
2 tablespoons granulated sugar
1/2 teaspoon vanilla extract
1/4 cup all-purpose flour
1/4 teaspoon baking powder
Pinch of fine salt
1 small strawberry, diced, plus more for topping
Whipped cream, for serving (optional)
Place the butter in a 12-ounce or larger microwave-safe mug and microwave until just melted. Add the milk, sugar, and vanilla and whisk to combine. Add the flour, baking powder, and salt, and whisk until well-combined. Add the strawberry, then gently push the pieces down so they're mostly submerged in the batter.
Microwave on high for 60 seconds, and then in 15-second bursts until the cake begins to pull away from the sides of the mug and the top looks set, dry, and springs back when gently touched, about 30 to 60 seconds more. Top with extra strawberries and a dollop of whipped cream and serve immediately.