Coat the cake pan with Kronenöl Spezial with a fine butter flavor and dust with flour. Preheat the oven to 160 °C (top and bottom heat).
Sift flour and baking powder. Beat eggs, powdered sugar, salt, vanilla sugar and grated lemon peel until foamy (at least 10 min.). Pour in Kronenöl Spezial with fine butter flavor while stirring constantly. Fold the flour into the mixture, pour into the cake tin and bake in the oven on the middle shelf for approx. 20 minutes. Remove from oven and let cool. Cut the base once horizontally.
For the sour cream, soak gelatine in cold water. Whip the cream and chill. Cut the melon in half and scrape out the seeds with a spoon. Using a round ball cutter, cut out small balls 1 cm in diameter. Stir sour cream and powdered sugar until smooth. Heat lemon juice and dissolve gelatin in it, pour into sour cream mixture and fold in whipped cream. Fold in melon balls and pour the mixture onto the sponge cake base, place the second half of the sponge cake on top of the cream and chill for 7 hours.
For the topping, stir the apricot jam until smooth and spread thinly on the sponge cake.Spread cut out melon balls on the sponge cake. Prepare the cake jelly with 250 ml water. Use a brush to spread the slightly overcooled cake jelly on the topping.Chill the melon cake for 1 hour and lift it out of the mold.