Florida Grapefruit Sponge Cake

For the Florida Grapefruit curd:

1. Start with the curd. Zest one Florida Grapefruit and squeeze two grapefruit to extract the juice.
2. Separate the egg whites from the yolks. Mix half the zest with the egg yolks, reserving the other half for the sponge cake.
3. Mix the sugar, salt and cornstarch in a saucepan. Add the grapefruit juice and whisk until smooth.
4. Place the saucepan over medium heat and bring to a boil while whisking. Cook for 2 minutes, whisking constantly to avoid burning.
5. Pour a little of the hot mixture into the bowl containing the egg yolks. Whisk immediately to prevent them from cooking.
6. Transfer the mixture to the saucepan and whisk until fully incorporated.
7. While whisking, thicken over low heat for 5 minutes.
8. Remove from the heat and add the cold butter, cut into small cubes. Whisk until the butter is melted and well blended.
9. Place in a shallow dish, cover with cling film and leave to cool (2-3 hours).

For the sponge cake:

1. Preheat the oven to 350°F.
2. Separate the egg whites from the yolks. Whisk the yolks with the sugar until the mixture whitens. Then add the flour and baking powder and mix.
3. Melt the butter, then add it to the mixture with the vanilla extract and zest, previously reserved.
4. In a separate bowl, beat the egg whites until stiff with a pinch of salt. Gently fold them into the batter using a spatula, with slow movements to avoid breaking them.
5. Butter a round tin, pour in the batter and bake for around 45 minutes.

For the whipped cream:

1. Place the full whipping cream and mascarpone in a bowl of your food processor. Refrigerate them with the whisk for 1 hour to ensure they set properly.
2. Gently whip the cream and mascarpone to combine, then increase the speed. When the mixture thickens, add the sifted powdered sugar and continue beating until stiff.

To assemble the cake:

1. Cut the sponge cake in half horizontally. Spread half the cream over the first half, then cover with grapefruit curd.
2. Peel the remaining two grapefruit and remove the segments. Place half of them on top of the curd layer.
3. Place the second half of the cake on top. Repeat: spread a layer of whipped cream, a layer of curd, then add the remaining grapefruit segments for decoration.

Tip: Sprinkle grapefruit zest lightly over the top before serving.