Mix the cocoa powder with 50 ml of boiling water. Whisk eggs and sugar with the whisk of the hand mixer until creamy. Add the cocoa mass. Sift flour over the mixture and fold in. Put the sponge mixture into a greased springform pan (approx. 20 cm Ø), dust with flour, smooth it down and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see
manufacturer) bake for about 20 minutes.
Remove the cake, place it on a cake rack and carefully remove it from the edge with a knife. Let the cake cool down. Remove from the springform pan, wash and dry the cake. Put the cake back into the springform pan and sprinkle with 2 tsp rum.
Roughly chop the chocolate and melt over a warm water bath. Add syrup and vanilla extract and stir in. Leave to cool. Whip double cream and cream together and add 2 teaspoons of rum.
Pour some of the cream into the chocolate, stir and fold the mixture into the rest of the cream. Spread the chocolate cream on the base and chill for about 2 hours. Sprinkle with sea salt before serving.