I prefer white and pink grapefruits for baking, but the sweeter red grapefruits will also work.
Cake
1 cup (200 g) granulated sugar
1 teaspoon finely grated grapefruit zest (shiny yellow or pink part only)
1 cup (225 g) unsalted butter, at room temperature
2 large eggs
1/2 teaspoon vanilla extract
11/3 cups (190 g) all-purpose flour
1/4 teaspoon baking soda
1/2 teaspoon salt
1/2 cup (125 ml) buttermilk
1 tablespoon poppy seeds
Syrup
1/3 cup (80 ml) freshly squeezed grapefruit juice (from about 1/2 grapefruit)
1/3 cup (70 g) granulated sugar
Icing
1 tablespoon grapefruit juice
1/2 cup (65 g) confectioners’ sugar
Preheat the oven to 325°F (165°C). Butter a 91/2 x 51/2-inch (24 x 14-cm) loaf pan. Tear out a sheet of parchment paper. Fold it in half or thirds so that it can lie inside the whole width of the pan, ends hanging over the long sides of the pan. This paper hammock makes it easier to lift the cake out of the pan later.
For the cake, mix the sugar with the grapefruit zest in a small bowl. Rub the mixture together with your fingers so the zest releases its oils.
In the bowl of an electric mixer fitted with a paddle attachment, cream the butter with the sugar mixture until fluffy, 2 to 3 minutes. Add the eggs one at a time, beating well and scraping down the sides of the bowl as necessary. Add the vanilla and mix again.
Sift the flour, baking soda, and salt into a medium bowl. Add one third of the dry ingredients to the sugar mixture and mix on low speed just until combined. Alternate adding the buttermilk and the rest of the dry ingredients in halves to the sugar mixture, mixing until just combined and scraping down the sides of the bowl in between. Add the poppy seeds with the final addition of dry ingredients. Pour the batter into the prepared loaf pan.
Bake the cake for 55 to 60 minutes or until the top is golden and puffed, and a toothpick inserted into the center of cake comes out clean. Remove the cake from the oven and let it cool for 10 minutes.
Meanwhile, for the syrup, combine the grapefruit juice and sugar in a small saucepan. Bring to a simmer over medium heat, stirring to dissolve the sugar. Boil the syrup 1 minute and then remove it from the heat. With a toothpick or skewer, poke holes all over the top of the cake. Pour the syrup over the cake, a little at a time, brushing it with a pastry brush to ensure even soaking. When all of the syrup has been added, let the cake cool completely. To remove the cake from the pan, lift up the edges of the parchment paper and set the cake on a plate, then slide the paper out from underneath.
For the glaze, whisk together the remaining 1 tablespoon of grapefruit juice with the confectioners’ sugar until smooth. You want the consistency to be loose enough to drizzle, but not so thin that it soaks into the cake. If it’s too thin, add more sugar. If it’s too thick, whisk in some water a few drops at a time. Drizzle the icing over the top of the cake with a whisk (if it dribbles over the sides, all the better). The cake can be stored at room temperature covered in plastic wrap for 2 to 3 days.
Source: Adapted from my book, WINTERSWEET: Seasonal Desserts to Warm the Home
