Preheat the oven to 190°C/170°C fan/gas 5. Lightly butter and line two 20cm round sandwich tins.
Put all the ingredients for the cake into a large mixing bowl, add the tablespoon of milk and beat until smooth.
Divide the mixture evenly between the tins, smooth the top, and bake for 25-30 min until the cakes spring back when pressed.
Allow the cakes to cool in their tins for 10 min and then turn out onto a wire cake rack to cool.
Mix the topping sugar with the lemon juice. Prick the cakes and spoon the topping mixture over (put something underneath the cakes to catch the drizzle!).
For the filling, mix the mascarpone with a few tablespoons of the lemon curd until it tastes yummy and use this mixture to layer the cakes.