Preheat the oven to 325F/ 160C. Line your 9-inch springform cake pan with parchment, don't grease the sides and set aside.
Whip the eggs and sugar on high speed until they are white and more than triple in volume, then add the zest and juice of 1/2 lemon.
Sift the flour and salt together, add both to the egg mixture. In a small bowl melt a tablespoon of butter in the microwave and stir vanilla into it.
Spoon a cup of the batter into that bowl and stir everything together, then add this all back to the full batter and stir gently until blended, don't over mix as it will result in a cake that won't rise very much. Pour the batter into the prepared pan and bake for 25-30 minutes or until the centre of the cake springs back when gently pressed.
Cool the cake completely in the pan.
While the cake is cooling prepare the mousse.