The "Queen Esther" Cake

For the sponge cake and a 30x40 cm baking tray:
4 middle-sized eggs
2 egg whites
110 g of sugar
60 g of ground poppy
30 g of all-purpose flour
30 g of vegetable oil
1 tsp of baking powder
1 tsp of vanilla extract (or 8 g of vanilla sugar)
a pinch of salt
For the cream:
250 ml of heavy whipping cream
250 ml of milk
2 egg yolks
75 g of sugar
40 g of starch
80 g of white chocolate
1 middle-sized lemon
The cake is about 1 kg