1. Heat oven to 180C. Grease a 12 x 22cm loaf pan with butter and dust with flour. 
2. Whisk together butter and caster sugar until smooth. Beat in eggs, 1 at a time, incorporating well.
3. Sift together flour with baking powder and salt and fold in to the mixture in batches with milk to form a smooth batter. Add hazelnuts, the zest of 2 grapefruit and the juice of 1 grapefruit, and mix well.
4. Pour the batter into the pan and bake for 60 minutes or until a skewer comes out clean. 
5. Meanwhile make a glaze by heating the juice of 2 grapefruit with the 2 Tbsp of caster sugar until dissolved. 
6. Remove cake from oven and spoon over the glaze until it all soaks in. When cool, turn out cake, wrap in plastic wrap and rest at room temperature for 24 hours. 

Check out Warren's other grapefruit recipes

• Meaty grapefruit marinade
• Grapefruit curd
• Grapefruit mousse