1. Preheat the oven to 350°F. Lightly grease an 8" square or 9" round pan that's at least 2" deep.

  2. For the cake: In a large bowl, whisk together the flour, brown sugar, salt, baking soda, and spices. In a medium bowl, whisk together the molasses, oil, and apple juice (or cider).

  3. Stir the wet ingredients into the dry until evenly moistened. Stir in the crystallized ginger, then transfer the batter to the prepared pan.

  4. Bake the cake for 30 to 35 minutes, until a toothpick inserted in the center comes out clean with a few moist crumbs cleaning to it. Remove from the oven and cool completely before frosting.

  5. To make the frosting: In a large bowl, beat together the margarine, salt, and half the confectioners' sugar. Add the milk, mix until smooth, then add the remaining confectioners' sugar. Mix until incorporated, then add the rum and vanilla extract. Beat until fluffy and spreadable. Frost cake and sprinkle with diced ginger.
