Preheat oven to 350°F, and prepare an 8” x 5” loaf pan.
Add the sugar and grapefruit zest to a large bowl. Combine until the mixture resembles a grainy paste. You can use your hands to rub it together.
Beat the mixture as you add the eggs, grapefruit juice, and butter.
Add the smashed banana, and combine until thoroughly blended.
In another bowl, sift together the flour, salt, and baking powder.
Combine the sour cream and milk in a small bowl and whisk until thoroughly blended.
Add about a third of the flour mixture to the egg/grapefruit mixture, stirring continuously as you do.
Now add about a third of the sour cream/milk mixture, continuing to stir.
Repeat until you have incorporated all the ingredients into one smooth batter.
Transfer the batter into the loaf pan. Bake for about 45 minutes, or until a toothpick inserted into the middle comes out clean.
Allow the cake to rest for about 15 minutes before you transfer to a wire rack to cool.