1Place the butter in a 12-ounce or larger microwave-safe mug and microwave until just melted. Add the milk, sugar, and salt and whisk to combine. Add the flour, baking powder, and vanilla and whisk until well-combined. Add the diced strawberry, then gently push the pieces down so they're mostly submerged in the batter.
2Microwave on high for 60 seconds, then in 15-second bursts until the cake begins to pull away from the sides of the mug, the top looks set and dry, and springs back when lightly touched — about 30 to 60 seconds more.
3Top with extra strawberries and a dollop of whipped cream and serve immediately.
Notes
Best served immediately. Use a large enough mug to allow the batter to rise without overflowing. Quick homemade whipped cream can be made by shaking some cream in a mason jar.