Whisk the sugar and juice in a medium microwave safe bowl. Whisk in the eggs, making sure to get them completely incorporated.
Microwave for 1 minute, then whisk. Microwave for another minute, then whisk. Repeat microwaving for 30 seconds and whisking until the curd has thickened. It will get lighter in color and be like a thin pudding. Cool completely.
This cake does well in the fridge- the flavors meld, and the toffee stays crispy and crunchy.
This is a 2 layer 6.5" cake, but you can do make a 3 layer 8" cake. Double everything EXCEPT the lemon curd. If you want a 6.5" cake, you're going to have leftover lemon curd, or you can deal with measuring 1 1/2 eggs to get the amount you need (and start checking the curd earlier - there's going to be less in the measuring cup, so it may cook faster than a full batch).