In a microwave-safe mug (12 ounces or larger), whisk together the flour, sugar, baking powder, and salt until evenly combined with no lumps.
Add the egg yolk, milk, oil, and vanilla extract to the dry ingredients. Stir gently with a fork just until the batter is smooth and no dry flour remains, about 10-15 seconds.
Fold in the diced strawberries, distributing them throughout the batter. Tap the mug gently on the counter to release any air bubbles.
Microwave on high power for 55-65 seconds, checking at 50 seconds if your microwave runs hot. The top should look just set with a few slightly moist spots in the center.
Let the mug cake cool for 30 seconds before eating — it will firm up to perfect texture. Enjoy straight from the mug or invert onto a plate.
Optional: Top with whipped cream, ice cream, extra strawberry slices, or a dusting of powdered sugar while still warm.
