Cake
1 cup butter
1/2 cup shortening
2 cups granulated sugar
5 eggs
3 cups all-purpose flour
1/2 teaspoon baking powder
1 cup milk
2 tablespoons rum
1 tablespoons Key lime zest
2 teaspoons Key lime juice
1 1/2 teaspoons vanilla extract
1/2 teaspoon lemon juice

Preheat oven to 325 degrees F (160 degrees C). Grease and flour a 10-inch tube pan.

Mix together the flour and baking powder.

In a large bowl, cream together butter, shortening and sugar until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir remaining ingredients. Pour batter into prepared pan. Bake for 50 to 60 minutes, or until a wooden pick inserted into the center of the cake comes out clean.

Meanwhile, make the glaze.

Glaze
1/4 cup granulated sugar
1/4 cup butter
2 tablespoons Key lime juice
3 tablespoons rum

Combine sugar, butter and Key lime juice in a small saucepan. Let it boil for a minute or two, stirring constantly. Remove from heat and stir in the rum.

Allow the cake to cool in pan 10 minutes. Turn it out onto wire rack. While warm, prick the top of cake in many places with a wooden pick. Pour the glaze over the warm cake. Cool completely before serving.