Whisk the eggs and sugar in a mixer until stiff. Sieve together the flour and potato starch. Then, using a spatula, gently fold it into egg and sugar mixture. Pour it into a 9'' spring form cake tin, buttered and floured. Bake at 325° F for 45 minutes. Remove from the oven and let cool. Once it is cooled completely, slice the cake into four layers.
To Make the Custard.
Bring the milk with the vanilla and lemon zest to a boil. Remove from heat and strain. In a separate bowl, whisk the eggs and yolks together with the sugar. Add the flour, and mix well. Once the mixture is smooth, temper it with a bit of warm milk, stirring while adding. Continue to pour the remaining milk, mixing it well with the flour and egg mixture. Return the mixture to the heat and cook over a low heat for 5 - 10 minutes stirring continuously, allowing it to boil for 2 - 3 minutes. Remove the custard from the heat and allow it to cool.
To Make the Whipped Cream.
Whisk heavy cream and confectioners' sugar until stiff peaks form. Once the confectioner's custard is cold, fold it in the whipped cream.
To Make the Syrup.
Bring water and sugar to a boil, and cook for 3-4 minutes. Turn off the heat and mix in the rum and the orange juice.
Assembling the Cake.
Line a medium size glass mixing bowl with plastic wrap. Line with one of the top layers of the sponge cake, brush with the rum/orange juice syrup. Pour in 1/3 of custard/whipped cream mixture and spread evenly. Top with another sponge layer and brush with the rum/orange sauce. Pour another 1/3 of the custard/cream on the sponge layer and spread evenly. Top with the bottom cut of the sponge cake to form a tight lid and moisten with rum /orange juice syrup. Set in the freezer and let set for 2-3 hours.
In the meantime cut the remaining sponge cake in small cubes. Invert the cake on a serving plate and gently remove the plastic wrap. Spread the remaining custard cream mixture evenly all over the cake to seal and make a smooth dome. Pressure the cake cubes onto the custard dome to cover completely. Chill the cake in the refrigerator until ready to use. Dust abundantly with powdered sugar and serve.