In a microwave-safe mug, whisk together melted butter and sugar with a fork.
Add egg yolk, milk, lemon juice, and vanilla. Stir until smooth.
Add self-raising flour and mix until just combined.
Drop 1 tbsp lemon curd in the center of the batter. Do not stir.
Microwave on high (1000W) for 50–70 seconds until set but soft.
P id=”instruction-step-6″>6. Let cool for 1 minute. Serve warm with optional toppings.
