Heat oven to 325°F. Grease a 9" x 5" loaf pan and line it with parchment paper; set aside.
Combine sugar and grapefruit zest in the bowl of a stand mixer. Using your fingers, rub together until sugar turns yellowy-orange and you smell that citrusy goodness.
Connect paddle attachment, then add butter to bowl. Beat on medium-high until light and fluffy.
In a liquid measuring cup, beat eggs together. Gradually pour eggs into butter mixture (about a teaspoon at a time), beating well after each addition. You want to do this slowly; it may take about 5 minutes to fully incorporate the eggs.
In a large bowl, whisk together flour, baking powder, and salt.
Add a quarter of the minced rosemary to butter mixture. Sift one-fourth of the flour mixture on top, then gently fold in with a silicone spatula. Repeat thrice more.
Spread batter into prepared pan, smoothing top with an offset spatula or the back of a spoon.
Bake for 55–65 minutes. Cake is done when a wooden pick inserted in the centre comes out clean with only a few fluffy crumbs attached.
Transfer pan to wire rack. Allow cake to cool for 10 minutes.
While cake is cooling, mix together grapefruit cordial and grapefruit juice.
Poke warm cake quite a few times with a toothpick, then slowly drizzle grapefruit syrup all over.
Cool cake completely, then remove from pan.
Dust cake with confectioners’ sugar and top with candied grapefruit slices before serving.
Notes
If you don’t have caster (AKA superfine) sugar, very briefly blitz granulated sugar in a spice grinder or blender. Don’t grind too much, otherwise you’ll end up with confectioners’ sugar!
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