• Preheat the oven to 350 F (180 C) for a conventional oven. Lower the temperature by 20 F (15 C).
• Grease a 9 x 5 inch loaf pan. Line with parchment paper on the long side, overhanging on the side
  

Prepare the cake batter:

• Rinse the grapefruits clean with water and pat dry. Zest the skin, be careful not to zest the white part of the skin because they are bitter. You will need about 1 Tbsp of zest for the cake and about 1 tsp for the glaze
• After zesting the fruit, cut it and start juicing. You will need 120 g of juice (1/2 cup) for the cake and reserve about 1 Tbsp (you may not use all) for the glaze
• In a mixing bowl, add 1 Tbsp of the zest to the sugar and rub it with your fingers for about 30 seconds or so. They would resemble wet sand
  
• 
• To the same bowl, add 3 eggs, sour cream, and the juice. Whisk well until the mixture is emulsified
  
• Combine the all-purpose flour, baking soda, baking powder, and salt. Add this to the egg mixture and fold using a rubber spatula until just combined.
  
• As long as you don't see pockets of dry ingredients, you are done mixing. Do not overmix
  
• Scrape the batter into the prepared pan. Tap the pan on the counter a few times to pop any air bubbles and to even out the batter in the cake pan so the cake won't be lopsided later
  

Baking:

• Put this in the middle rack and bake for 65-70 minutes. Start checking at 65 minutes. A cake tester inserted into the center of the cake should come out clean, with a bit of sticky crumb is fine as long as it's not wet
  
• Set the pan on a cooling rack. Let the cake cool in a pan for 5 minutes. Then use an offset spatula to loosen the cake and then carefully lift it out of the pan and put it on a cooling rack to let it cool down completely. The cake needs to be cooled down completely before glazing

Glaze the cake:

• Only prepare the glaze when you are ready to glaze them as they set pretty quickly. Combine the powdered sugar with 1 tsp of reserved grapefruit juice you set aside earlier. If it's too thick, you can add a bit more of the juice to thin it out. Spread the glaze on top of the cake. The glaze will set in few minutes
  

To serve:

• The cake is very moist and stays moist for days if kept in an air-tight container.

How to store:

• They can be kept at room temperature for 2-3 days. For longer storage, this cake also freezes very well. Wrap few slices together in a cling wrap and put in a freezer bag, push all the air out and seal the bag. They can be kept frozen for 1 month. Simply thaw at room temperature for few hours before serving.