Grease the oven pan (32 x 39 cm) and sprinkle with breadcrumbs. Cream eggs, 250 g sugar and 1 packet of vanilla sugar with the whisk of the hand mixer. Stir in oil and fruit juice.
Mix flour and baking powder, stir in portions. Pour the dough into the fat pan and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) on the 2nd rack from below for 15-20 minutes.
Let it cool down.
Drain large tins of apricots in a sieve, collecting the juice. Soak gelatine in cold water. Puree almost half of the apricots, dice the rest. Mix the apricot puree with 375 ml apricot juice, buttermilk, 120 g sugar, 2 packets of vanillin sugar and lemon juice.
Squeeze the gelatine and dissolve with 3-4 tbsp. apricot juice over low heat, then stir into the buttermilk. Put it in a cold place.
Place the angular baking frame around the sponge cake base in the fat pan. Whip 400 g cream until stiff. As soon as the buttermilk starts to gel, fold in first the cream, then the apricot cubes. Pour the mixture into the baking frame and spread.
Chill for at least 2 hours.
Whip 250 g cream until stiff and fill into a piping bag with star-shaped spout. Drain the apricots from the small tin and cut 9 apricot halves in half. Use the rest of the apricots for other purposes. Cut the cake into 18 slices and decorate each with a cream tuff and an apricot wedge.
Warm the jam with 1-2 tablespoons of water in a small pot. If necessary, pass through a sieve. Sprinkle some jam on each cream tuff. Sprinkle with chopped pistachio nuts of your choice.
Tip: If you prefer to keep the cake cold, cut the bases in half, surround both halves with a frame of extra-strong aluminium foil and then pour in the buttermilk mixture.