1. Preheat the oven to 350°F (180°C). Line a 10-inch (26cm) springform tin with baking parchment and set aside. Remove the curd (quark) from the fridge and let it come to room temperature while the sponge bakes or cools down.

2. Add the eggs to a large bowl and whisk until slightly foamy. Add the sugar gradually while whisking on high speed until the mixture looks pale and thick (do not overbeat the eggs or the cake will probably collapse when baked). Add the vanilla extract and stir in. Mix the flour, cornstarch, and baking powder. Sift a small portion onto the egg mixture and fold in very gently. Repeat the process several times until all of the flour mix is incorporated. Pour the batter into the prepared tin, give it a small shake to allow air bubbles to rise up, and bake for about 18-20 minutes. The sponge should have risen nicely with a golden brown color. Take out of the oven, loosen the sponge from the edge of the springform tin with a knife and let cool down completely.

3. For the filling let the gelatin leaves soak in a bowl with cold water for about 5-8 minutes. Whisk the (cold) heavy cream until stiff peaks form and set aside. Add the quark and confectioners’ sugar to a large bowl. Add the lemon zest and juice, the orange zest, and limoncello (optional) to the bowl and mix in. Squeeze the gelatin to remove some water and add to a small saucepan. Heat up gently until the gelatin is dissolved completely. Gradually add some of the curd mixture and mix to combine. Remove from heat and stir in some more curd mixture, then transfer to the large bowl and mix until well combined – if the temperature differences are too great, the gelatin may clump. Gently fold in the whipped cream in 2-3 batches. Pour the mandarin orange slices into a sieve and drain well.

4. Remove the top of the sponge cake if is domed or sunken in so you get a straight surface, then cut horizontally in half. Lay one of the cakes on a serving plate and place a cake ring tight around it. Add about 1/3 of the curd mixture on top and spread evenly. Place half of the mandarin orange slices on the filling and top with another 1/3 of the cream. Repeat this step with the remaining mandarin orange slices and finish with the last 1/3 of the filling. Smooth out the top and place the second cake layer on top inside the cake ring. Place the cake in the fridge for at least 4 hours (or overnight). Before serving, use a sharp knife to divide the top sponge cake into 12 pieces – try not to cut into the filling. The cuts are just to prevent the filling from being compressed too much when you try to cut into the cake later on. Carefully remove the cake from the ring, dust with confectioners’ sugar, and serve.