In a microwave-safe mug (10–12 oz), whisk together flour, sugar and baking powder
Stir in milk, oil, vanilla extract and lemon juice until smooth and well combined.
Gently swirl in 1 tablespoon of lemon curd. You can leave it swirled or drop it into the center for a gooey surprise.
Microwave on high for 1 minute and 15 seconds to 1 minute and 30 seconds, depending on your microwave. The cake should rise and look set but still slightly moist.
Let it cool for 1–2 minutes. Top with extra lemon curd or a dusting of powdered sugar, if you like.