In mixer or food processor, mix the peels of grapefruit and orange with the buttermilk until the peels are finely chopped. Add sugar, salt and butter and beat well.
In a bowl, mix flour, baking powder and baking soda. Add the mixture of buttermilk and mix until the mixture is wet, but do not mix too much.
Spread the mixture in 12 greased muffins tins. Cook in a preheated oven of 375 F 20 to 25 minutes or until a toothpick inserted in the muffins comes out clean. Let cool on a rack for about 5 minutes. Remove from the pan and let cool completely. (They will be good up to 3 days in the refrigerator and up to 1 month in the freezer).