Grease a round casserole dish (24 cm Ø; 7 cm high). Heat the sugar and 200 ml water in a pan and simmer for 20-30 minutes until the sugar caramelises to a golden yellow colour. Pour on the bottom of the mould and let it cool down.
Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Mix condensed milk, 400 ml milk and 8 eggs. Pour into the mould. Fill the fat pan (deep baking tray) to approx. 2⁄3 with hot water. Put the mould in. Let it stand in the hot oven for about 50 minutes.
About 20 minutes before the end of the cooking time, chop the chocolate and melt it in a hot water bath. Stir in oil. Mix flour and cocoa. Beat 3 eggs and brown sugar with the whisk of the mixer for about 5 minutes until thick and creamy. Stir in the chocolate mixture. Stir in the flour mixture and 4 tbsp. milk briefly. Fold in grated chocolate.
When the egg mixture has set, carefully place the dough on top and spread. Bake at the same temperature for about 20 minutes.
Remove from the oven and let it cool down. Loosen the edge all around with a knife. Chill for at least 6 hours, preferably overnight. Then turn the flan over onto a plate.