Soak the golden raisins in rum for thirty minutes to an hour.
Preheat oven to 350 degrees (F).
In a mixer on low speed, blend together the cake mix, water, and eggs. Turn off the mixer and add the rum and raisins. Set the mixer to medium and blend all the ingredients for 2 minutes.
Pour the batter into 2 greased 8-inch cake pans.
Bake for 20 - 30 minutes, making sure to check at 20 minutes because the layers cook faster with liquor substituted for water as the batter liquid. If you insert a toothpick into the middle, it should come out clean. Do not overbake! The cake will dry out.
Cool cakes in pans for 5 minutes then turn out onto a wire rack.
Let the cake layers cool before frosting.
Cream the butter with the vanilla, rum, salt, and 1 tablespoon of milk.
Slowly add the confectioners' sugar, blending it well with the other ingredients. If you need to thin the frosting, add another tablespoon or two of milk - or rum. Add additional confectioners' sugar if frosting needs thickened.
To assemble the cake, slice a thin layer off one of the cake layers so that it will be level on the bottom, then invert the cake layer onto a plate so that the flat side - the bottom - is facing up. (Feel free to eat the thin slice!) Frost this layer.
To add the second layer, invert it onto a plate, then place it on the frosted layer with the flat side down. (This is how you get a professional-looking cake - placing flat side to flat side, that is, bottom of the layer to bottom of the layer. Frost the rest of the cake.