Dice the strawberries into small, even 1/4-inch pieces—this ensures they distribute evenly throughout the cake and cook at the same rate. Place 2 tablespoons of butter into a microwave-safe mug and microwave on high for 20-30 seconds until fully melted. Tilt and rotate the mug to coat the sides and bottom with the melted butter; this prevents sticking and creates a golden crust.

In the buttered mug, combine the flour, sugar, baking powder, salt, and cinnamon. Stir these dry ingredients together until evenly mixed. Add the milk and vanilla extract, then fold everything together until just combined—a few small lumps are fine and actually help create a tender crumb. Gently fold in the diced strawberries last to avoid crushing them and distribute them evenly throughout the batter. I find that folding rather than stirring vigorously helps keep the strawberry pieces intact and prevents them from turning into a mushy pulp.

Microwave the batter on high for 90 seconds to 2 minutes, checking at the 90-second mark. The cake is done when a toothpick inserted into the center comes out clean or with just a few moist crumbs—it will continue to cook slightly from residual heat. Let the mug cool for 1 minute on the counter before eating, which allows the cake to set and makes it easier to handle. I always let it rest this full minute because diving in too early can result in an overly hot, slightly gummy texture that hardens as it cools.