• Preheat oven to 350° F. Spray a regular 8 ½ by 4 ½ loaf pan with nonstick spray.
• In a large bowl add sugar and grapefruit zest smash the zest into the sugar to release all the oils. Add flour, salt, and baking powder and stir.
• In a separate medium bowl, combine the buttermilk, oil, eggs, grapefruit juice, and vanilla extract.
• Add the flour mixture to the wet ingredients and stir until fully combined. Add the sour cream and gently stir in. Pour the batter into the loaf pan.
• Bake for 35-45 minutes. Cool on a rack in the pan for 10 minutes. Loosen a little bit from the sides and remove loaf from pan. Let cool completely on wire rack.

For the glaze

• In a small bowl, combine powdered sugar and grapefruit juice, and whisk until smooth. Drizzle the glaze over the cooled cake.