Chop the chocolate, melt it over a hot water bath, let it cool down. Separate eggs. Cream butter and chocolate. Stir in egg yolk and sugar bit by bit. Mix flour, cocoa and baking powder, sieve onto the mixture, stir in carefully. Beat the egg whites until stiff, adding icing sugar and salt.
Stir in 1/3 of the egg whites, fold in the rest. Grease the fat pan (approx. 32 x 39 cm) of the oven well and sprinkle with flour. Add the dough and spread evenly. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 25 minutes. Let the base cool down. Divide the base into 4 pieces, cut each base in half horizontally once. Warm up the jam and pass through a sieve. Spread 1/3 of the jam on the lower cake layer, place the upper layer on top. Cut approx. 6 slices each (approx. 8 x 5 cm) from the pieces. Chop the dark and milk chocolate, melt separately over a hot water bath. Cover the slices on the underside with the dark chocolate coating. Coat all around, except the underside, with the remaining apricot jam, let dry for about 30 minutes.
Spread 1/3 of the jam on the lower cake layer, place the upper layer on top. Cut approx. 6 slices each (approx. 8 x 5 cm) from the pieces. Chop the dark and milk chocolate, melt separately over a hot water bath. Cover the slices on the underside with the dark chocolate coating. Coat all around, except the underside, with the remaining apricot jam, let dry for about 30 minutes. Coat with the remaining dark chocolate coating and leave to dry. Pour the full-mich couverture into a small piping bag. Decorate the slices with it, let them dry.