Preheat the oven to 200°C (Gas Mark 6).
For the fondant: whisk the eggs with the sugar until mixture has doubled in volume. Melt the butter with the chocolate in a glass bowl over a pan of simmering water. Once melted, mix with the eggs and sugar, then carefully fold in the flour. Line the pastry rings with non-stick baking parchment. Pour the mixture evenly into the pastry rings. Bake in the oven at 200°C for 8 minutes.
For the sauce: remove the zest of the grapefruit. Boil the milk with the zest. Mix the egg yolks with the sugar . Pour the hot milk on to the eggs and return to the pan, cook, stirring constantly with a spatula, to 84°C (a good way to test that the sauce is ready is to run your finger along the back of a spatula; it should leave a mark when the sauce is cooked). Peel and segment the grapefruit. Cut the segments into small cubes and add them to the cold sauce.
Serve with the fondant. .
«If your custard is overcooked, blend it for 30 seconds.»