To make TWO sponge cakes, preheat the oven to 355°F (180°C). Prepare two 8-inch (20 cm) cake pans or springform pans. Butter and line the pans' bottoms and sides with parchment paper. Using a flour sifter, sift all-purpose flour with potato starch and set aside.

Place the eggs, egg yolks at room temperature, and sugar in the large mixing or bowl of a stand mixer fitted with a which attachment. Beat at medium speed for 10-15 minutes until the mixture doubles in volume and becomes whitish.

Add dry ingredients and gently mix with a rubber spatula, trying not to deflate the batter. Pour the batter into two prepared tins, equally devising the batter. Bake in the middle of the oven for 20-25 minutes.

Pro tip: If you can't bake two sponges simultaneously, whip up two sponges separately: prepare the sponge batter twice and bake each cake separately for optimal results.

Make sure the sponge cakes are done by testing them with a toothpick. If it comes out dry, the cakes are ready. Take the cakes from the oven, unmold them, remove the parchment, and let them cool completely on a cooling rack.

To make sugar syrup, pour water and sugar into a small saucepan and bring to a boil until the sugar dissolves. Remove from the heat, add liqueur, and let the syrup cool down.

To make Diplomat cream, first, make a pastry cream. In a mixing bowl, mix egg yolks and granulated sugar with a hand whisk. Add cornstarch and mix again. Pour milk, cream, and the scraped vanilla seeds into a small saucepan and bring them to a boil.

Add the hot milk mixture to the egg yolk mixture, constantly stirring. Pour the preparation back into the saucepan and cook over medium-low heat, constantly stirring, until the cream thickens.

Remove the cream from the heat and transfer it to a shallow dish. Cover with plastic wrap in contact with the cream's surface to prevent skin formation. Let the cream cool at room temperature without chilling in the refrigerator.

To make the whipped cream, pour the cold cream into the bowl of a stand mixer fitted with a whisk attachment. Whip it at medium speed into soft peaks (not stiff peaks).

For the Diplomat Cream, place the pastry cream in a mixing bowl and whip with a hand whisk a few times. Add 1/3 of the whipped cream into the pastry cream and stir vigorously with a whisk. Then add the remaining whipped cream and gently mix with the hand whisk or a rubber spatula from the bottom to the top. Transfer the cream to a pastry bag.

To assemble the Mimosa cake, use a long sharp knife to remove the dark outer crust of the sponge cakes. With a large serrated knife or a cake leveler, cut each cake into two sponge disks: you will obtain four layers of the cake.

Cut one of the sponge cake discs into 8-10 mm wide strips and then into small cubes with 8-10 mm sides. Keep the cake cubes and crumbles for further decoration.

Place the bottom layer on a cake board or a serving cake plate and soak it with the sugar syrup using a silicone or pastry brush.

Pro tip: For a neat appearance, assemble the cake in a high cake ring (or a mousse cake mold) lined with an acetate strip.

Use the pastry bag with a cut tip (you can also use a spoon) to spread 1/3 of the cream on the first cake layer. Cover with the second layer of sponge cake and soak it with the syrup. Cover it with another 1/3 of the Diplomat cream. Top it with the last sponge cake disc, and soak it with the syrup. Remove the cake ring if used. Coat the top and sides of the cake with the remaining cream using the rubber or offset spatula.

Cover the entire cake with the sponge cake cubes kept aside. Sprinkle it with the remaining sponge crumbs. Refrigerate for 3-4 hours before serving. Dust the top of the cake with icing sugar if desired.