Grease a microwave-safe pudding bowl (about 1.2 litre capacity).
Sift the flour and baking powder together.
In a food processor, blend butter, sugar, eggs, flour, and baking powder into a smooth batter. Alternatively, cream butter and sugar, then beat in eggs and dry ingredients.
Add lemon zest and juice, mixing well. Stir in milk until the batter reaches a soft dropping consistency.
Spoon the mixture into the prepared bowl and smooth the top.
Cover with cling film or a microwave-safe lid.
Microwave on high for 3–4 minutes until the sponge is set and springy.
Let it stand for 2–3 minutes before carefully turning out.
Serve the Microwave Syrup Sponge warm with golden syrup or jam and optional custard.