Preheat oven to 375º
Grease and flour (or fine sugar) two 9" cake pans
Bake 25 minutes, or until toothpick comes out clean
Whisk flour, baking powder, and salt.
Combine juices and water.
Cream butter and sugar; add eggs, one at a time; add zest; beat until blended.
Alternately add dry mixture and juice mixture to the butter/sugar mixture, starting and ending with dry ingredients; blend well.
Cook all ingredients in a double boiler (or steel bowl over simmering water in saucepan) until thick; cool.
With a hand mixer, beat mixture as it cooks until thick and proper consistency to spread; cool.