Zest 1 grapefruit into 200 g caster sugar, and whisk well. Cover with a plate, and leave to rest for 1 hour. We do this to capture the grapefruit aromas in the sugar. Note: You can make this the day before, and leave it to rest in the fridge overnight.
2: Squeeze the grapefruit and set the juice aside.
3: Melt 70 g butter over low heat, then remove from heat and allow to cool.
5: In the bowl of a mixer, pour 3 eggs, 3 tablespoons grapefruit juice, and 100 g liquid cream. Add the sugar + zest, and whisk until smooth and frothy.
6: Gradually add the flour + baking powder mixture.
7: Finish by gently pouring in the cooled melted butter.
8: The mixture should be well blended.
9: Pour into a buttered cake tin. If possible, leave to rest in the fridge for 1 hour.
10:
Grapefruit syrup
Preheat oven to 180°C (360°F). Pour 50 g water and 75 g caster sugar into a small saucepan, bring to the boil and stir well.
11: Remove from the heat, add the rest of the grapefruit juice and stir. Your syrup is now ready, cover it while you wait.
12:
Baking
If you want a nice opening when baking, slit the top of your cake like this.
13: Bake for around 30 to 40 minutes at 180°C (360°F), checking 5 minutes before the end to be sure.
14: Turn out the warm cake onto a wire rack set on a plate, and drizzle with the grapefruit syrup. Collect the excess syrup on the plate, and pour it over the cake several times, or better still, brush the syrup over the cake.
15: Leave to cool before serving.
Remarks
You can add 1 or 2 tablespoons of rum to the grapefruit syrup, to taste. The cream can easily be replaced by mascarpone.