Preheat oven to 350˚F, and prepare three 6" cake pans by buttering their bottoms and lining with parchment paper.
Combine the sugar, eggs, and lemon zest together in the bowl of a standing mixer, fitted with a whisk attachment, until the sugar is dissolved into the eggs. Continue to mix on medium-high for a few minutes until it lightens in color, and thickens.
In a separate bowl, whisk together the flour, potato starch, baking powder, and salt. Using a rubber spatula, fold the flour mixture into the eggs and sugar one-third at a time, making sure it's well mixed before adding the next third.
You should have about 3 cups of batter. Divide the batter between the three prepared cake pans, and bake until lightly browned, about 12-13 minutes. Run a sharp knife around the inside of each cake pan, and flip the cake layers out onto a wire rack to cool. Leave the parchment paper on the layers. Cool, wrap in plastic, and leave on the counter overnight, or at least for several hours.
Make the custard / pastry cream, and warm the jams for easy spreading.
Whip the cream to soft peaks, add the sugar one tablespoon at a time, and continue to whip to medium-firm.
Remove the parchment paper from the cake layers, and spread custard / pastry cream on the tops of two of the layers. Lightly spread the apricot jam on one of the two custard topped layers, and on the layer without custard. Lightly spread the raspberry jam on the other custard topped layer. Gently stack the cake layers, ending with the cake layer without any custard.
Gently scrape any excess custard and jam dripping between the layers. Top the cake with whipped cream, and using a rubber spatula apply a very thin coating of whipped cream to the sides.