1Preheat oven to 325 degrees.Grease and flour a 10 inch tube pan.
Stir the flour and baking powder together in a mixing bowl.
Cream the shortening with 1 cup butter and 2 cups sugar in a large mixing bowl until light colored and fluffy. Beat in the eggs, one at a time until mixture is light colored. Stir in 2 tablespoons rum, 2 teaspoons Key lime juice, Key lime zest, and vanilla extract until thoroughly blended. Stir in the flour mixture alternating with the milk. Pour the batter into the prepared pan.
2Bake in preheated oven until a toothpick inserted into the center comes out clean, about 90 minutes.
Meanwhile, make the glaze by creaming 1/4 cup sugar with 1/4 cup butter in a small saucepan until light colored and fluffy. Stir in 2 tablespoons Key lime juice until evenly blended. Bring to a boil, stirring constantly. Remove from the heat and stir in 3 tablespoons rum.
3While cake is still in pan, poke holes in cake and drizzle warm butter glaze all over the top. Let sit in pan for 5 min. Then turn out onto platter.