Make your lemon curd up to 1 day ahead of time, or a few hours before baking the sponge. To make lemon curd, combine egg yolks, sugar, cornstarch, salt, and lemon zest in a medium mixing bowl and whisk together well to combine.
Add in lemon juice and whisk well.
Transfer the mixture to a pot over medium low heat. Heat the mixture until it thickens, whisking constantly to avoid lumps. It should reach the consistency of a thick custard. Add in the three tablespoons of butter and stir to combine.
Transfer to a bowl and place plastic wrap so that it touches the surface of the curd, preventing it from forming a skin. Put in the fridge and let cool completely.
Preheat your oven to 390 F (200 C).
Add your eggs and sugar to the bowl of a stand mixer. Beat until light and fluffy and tripled in size (about 5 minutes). Add your vanilla extract and mix to combine.
Meanwhile, measure out your flour, baking powder, and salt and whisk to combine.
Sift your dry ingredients into your egg mixture in two parts, folding gently to combine. You can also sift in the ingredients and run the mixer on very low speed to incorporate gently without knocking air out. We would still recommend folding a few times at the end to make sure the dry ingredients are fully incorporated!
Spread your batter onto a parchment lined rectangular baking sheet, aiming for about half an inch thickness.
Bake for 6-8 minutes – make sure not to overbake! Check it right after 6 minutes by poking a knife or toothpick into the sponge and making sure it comes out clean. Overbaking will make this sponge tough.
Once your sponge is baked, let it cool for just a few minutes and then turn out onto a piece of parchment paper dusted with icing sugar or granulated sugar.
You want to do this while the sponge is still warm, so it doesn’t crack later!
Roll up your sponge and sugared parchment paper along the short edge of the sponge.
Let it cool completely in its rolled up shape.
(This step is optional if you would like to skip the whipped cream!) While the sponge is cooling, make whipped cream by combining heavy cream, sugar, and a splash of vanilla extract in a stand mixer with the whisk attachment and whipping until it reaches stiff peaks. You can also whip by hand, or using a handheld whisk.
Once it has cooled completely, unroll and spread lemon curd across the sponge all the way to the edge.
(Optional) Spread whipped cream over the lemon curd.
Gently roll up the sponge the same way that it was rolled when cooling. It's okay if some of the filling falls out at the very end!
Place the roll seam side down on a cutting board/serving platter. Decorate the top with any leftover whipped cream and lemon zest. Slice, serve, and enjoy!