Heat oven to 325 degrees F (160 degrees C). Grease and flour a 10-inch
Cake: 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: 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 cake 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.