Chop the chocolate, melt it in a hot water bath, let it cool down a bit. Line two baking trays with baking paper and preheat the oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer).
Mix 100 g butter, sugar and egg until creamy. Sift 240 g flour and 1 tbsp. cocoa over it and stir in briefly. Stir in the liquid chocolate. Then knead with floured hands briefly to a smooth dough.
Roll out the dough between two sheets of baking parchment approx. 4 mm thin. Cut out about 20 hearts with a heart cutter (approx. 8 cm Ø). Place them on the baking trays and bake in a hot oven for 10-12 minutes each. Take them out, let them cool down on the baking tray for about 5 minutes, then let them cool down on a cake rack.
For the filling, wash and dry the orange, and finely grate the peel. Halve the orange and squeeze 1 half. Sift 80 g icing sugar. Mix with 90 g butter and orange peel with the whisk of the mixer until white and creamy. Stir in 1 tsp. orange juice. Spread half of the hearts with the orange butter cream, place the remaining hearts on top.
For the chocolate icing, sieve 120 g icing sugar and 2 tablespoons of cocoa into a bowl. Mix with about 2 tablespoons of orange juice to a thick icing. Decorate hearts with it. Leave to dry and store in a cool place.