Prepare three loaf pans or 9" round baking pans by coating them with cooking spray or buttering them and dusting them with a coat of flour.
In a large bowl, cream the butter with an electric mixer until it is light and fluffy. Add the sugar gradually in several additions until it is completely incorporated. Beat thoroughly.
Separate the eggs. Set the whites aside, and beat the yolks until they are thick and pale. Beat them into the butter and sugar mixture.
In a separate bowl, beat the egg whites until they are very stiff. Fold them gently into the batter, being careful not to overmix.
Sift the flour with the mace, cinnamon, and baking soda. Mix the dry ingredients into the batter with a spatula.
Fold the milk, fruit, nuts, and citron into the batter.
Divide the batter evenly among the three pans. Cover each pan tightly with aluminum foil.
Set up boiling water in a steamer, and place the cake pans in the hot steamer. Steam for about 3 hours, adding new boiling water when necessary.
If you don't have a steamer, boil several inches of water in a large stockpot. Set a heatproof dish in the bottom of the stockpot and set the cake pans on top of the dish so that they don't touch the bottom of the pot (you may have to steam each cake separately). Cover the pot and steam for 3 hours, adding more boiling water when necessary.
Near the end of the steaming time, preheat the oven to 300 degrees Fahrenheit. When the 3-hour mark has passed, bake the cakes for 60 to 90 minutes or until they are dark and pass the toothpick test.
Cool the cakes completely before unmolding, cutting, and serving them.