Line the base of a 23 cm springform tin with baking paper, then lightly grease it all with a little oil.
Preheat the oven to 180°C.
Whisk the 14 egg yolks, 5 egg whites, vanilla, lemon zest and sugar for about 1 minute by hand until foamy but not firm. Whisk in the flour, baking powder and ground walnuts, then pour the batter into the prepared tin.
Bake in the oven for 20-30 minutes until just cooked through.
While the batter is cooking prepare the topping.
Put the egg whites, sugar and walnuts into a medium saucepan over medium heat. Stir the mixture constantly so it doesn’t catch and burn, for about 15 minutes until it’s thick, sticky and golden brown.
Pour in the lemon juice and keep stirring for a further 6-7 minutes. Turn off the heat and set aside.
When the bottom layer of the cake is cooked, remove it from the oven and reduce the oven temperature to 150°C.
Scoop the still-hot walnut mixture over the cake that just came out of the oven, spreading it evenly and gently pressing the walnuts down.
Put the tin back into the oven and bake for a further 30 minutes until the top is light brown. Allow the cake to cool completely in the tin before removing and slicing.
The cake freezes and defrosts perfectly. Simply wrap it in foil whole, half or quarters.