6 ounces (1 1/2 sticks) unsalted butter, at room temperature
1 tablespoon fresh grapefruit zest
3/4 cup granulated sugar
1 1/2 cups grapefruit curd
3 large eggs
1 3/4 cup plus 1 tablespoon self-rising flour
To make the grapefruit curd, in a small saucepan, simmer grapefruit juice. Simmer and reduce from 1 cup to 1/2 cup. Remove from heat and allow to cool slightly.
In a medium heatproof bowl (a metal bowl is best), stir together butter and granulated sugar until well incorporated and fluffy. I used a wooden spoon and some elbow grease. Using a whisk, add the egg yolks and whisk to combine. Add the eggs, one at a time, beating for one minute between each addition. Whisk in the salt and cloves and lemon juice. The mixture will seem slightly curdled. That’s ok. Slowly stream in and whisk the slightly cooled grapefruit juice until evenly combined.
In a small or medium saucepan (I used the same saucepan I used to reduce the grapefruit juice), bring a few inches of water to a simmer. Place the heatproof bowl over the simmering water , making sure that the bottom of the bowl does not touch the simmering water.
Whisk constantly. The sugar will begin to dissolve and the mixture will begin to thicken. This take between 8 and 10 minutes of near constant whisking. If you stop whisking, the eggs in the mixture may create lumps so keeeeeep whisking.
Once thickened to the texture of warm pudding, remove the bowl from the simmering water and transfer to a small bowl. Allow to rest in the refrigerator while you make the cake.
Place a rack in the center of the oven and preheat oven to 350 degrees F. Line the bottom of an 8-inch round baking pan with parchment paper, grease the pan and set aside.
To make the cake, in the bowl of an electric stand mixer fitted with a paddle attachment, or in a large bowl with a set of hand beaters, beat together butter, sugar, and grapefruit zest until pale and fluffy, 3 to 5 minutes. Add half of the grapefruit curd and beat to combine. Add the eggs, one at a time, beating for 1 minute between each addition. Add the flour and use a wooden spoon to fold together until thoroughly combined.
Spoon the batter into the prepared pan. Top with the remaining grapefruit curd and use a butterknife swirl the curd just slightly into the cake. Bake for 35 to 45 minutes until a skewer inserted in the center comes out with moist crumbs.
Allow to cool for 15 minutes before transferring to a wire rack to cool completely.