Preheat your oven to 350 F (175C). Butter your cake pan or spray oil on it. You need a 6-inch one.
Crack your eggs in a glass. Pour in another similar glass the same amount of sugar as eggs. Add the sugar in a bowl. Measure all-purpose flour 0.8 amount as the eggs are in volume. Add the rest as potato starch.
Whip the eggs and sugar to a foam. It takes 7-10 minutes with a hand mixer at low/medium speed. When a pattern stays on the foam for a moment it is ready.
Grate the lemon zest and add to the whip, add also the vanilla extract.
Mix the potato starch and flour together. Add it with a sieve. Mix gently to a smooth batter. Pour into the cake tin.
Cook on the lower rack in the oven for 35 minutes. Try if the cake is done with a toothpick. Let the cake cool before cutting it
Cut the cooled sponge cake into three layers. Use the top one as the base layer.
Whip your heavy cream. Flavor with vanilla.
Moisten your base with some milk. Add half of the strawberry jam and some cream, about half inch. Do the same for the second layer. Do not moisten the top layer.
Spread the rest of the whipped cream on the sides and top of the cake. Use a knife or palette knife. You can also pipe the cream into patterns.
Cut the strawberries into slices and add slices to the sides. Decorate the top with whole strawberries or ones cut as a fan shape.