Roughly chop the chocolate. Cut butter into pieces. Melt the chocolate and butter over a warm water bath, take it down.
Separate eggs. Mix the egg yolks, 50 g sugar and chestnut purée in a bowl and whisk until creamy. Mix flour and baking powder and add to the mixture. Slowly add the chocolate mixture and mix in thoroughly.
Beat egg white and 1 tablespoon sugar until stiff. Carefully fold the beaten egg whites into the chocolate mixture. Put the mixture into a greased springform pan (20 cm Ø) sprinkled with flour and bake in a preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 35-40 minutes.
Meanwhile peel the orange so that the white skin is completely removed. Cut oranges into thin slices. Take the cake out of the oven and let it cool down on a cake rack for about 1 hour. Heat the jelly in a small pot and take it down.
Spread orange slices on the cake and sprinkle with jam.