1 lemon, grated zest only
1 lime, grated zest only
3 tbsp treacle or golden syrup
1 tsp baking powder
115g/4oz caster sugar
1 nutmeg, grated, plus extra for serving
140g/5oz self-raising flour, sifted, plus extra for dusting
85g/3oz butter, well softened, plus extra for greasing
custard
2 free-range eggs, lightly beaten
1 tbsp milk, or more if needed