Line a baking tray (approx. 35 x 40 cm) with baking paper. Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Separate eggs. Beat the egg white and 3 tbsp. cold water until stiff, adding 100 g sugar.
Stir in the egg yolk briefly. Sift the starch, flour and baking powder over it and fold in.
Spread the sponge cake mixture evenly on the baking tray. Bake in a hot oven for about 10 minutes. Remove and turn out onto a clean tea towel sprinkled with 2 tablespoons of sugar. Carefully remove the baking paper immediately and let the sponge cake cool down.
Cut the sponge cake in three. Stir the jam until smooth. Knead marzipan with 2 tablespoons of liqueur (or juice). Roll out rectangularly (approx. 13 x 35 cm) on a work surface dusted with icing sugar.
Spread the marzipan plate with 1 tbsp. liqueur and place on a third of a sponge cake with the coated side down. Spread the other side of the marzipan with 1 tbsp. liqueur. Place the second third of the sponge cake on top and spread with jam.
Place the last third of the sponge cake on top. Weight the cake plate with a large board, chill for about 1 hour.
Chop dark and light chocolate separately. Melt dark chocolate coating and 20 g coconut oil, white chocolate coating and 5 g coconut oil separately in a hot water bath.
Cut cake edges straight. Cut the cake plate into approx. 24 cubes (approx. 4 x 4 cm). Cover Petits Fours with dark chocolate coating. Let dry on a cake rack. Pour white couverture into a disposable piping bag.
Decorate Petits Fours with it and let it dry. Insert the letter skewers.