Beat butter at medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add eggs and egg yolk, one at a time, beating until blended after each addition. Beat in vanilla and next 3 ingredients.
In a large bowl, combine flour and the next 3 ingredients; add to butter mixture alternately with whipping cream, beginning and ending with flour mixture. Beat at low speed until blended after each addition.
Grease the min bundt tins. Fill each tin two-thirds full of batter. Bake for 18 to 20 minutes or until a wooden skewer inserted in the center comes out clean. Cool in pan on a wire rack 15 minutes; remove from pan.
While the cakes are baking, make the Rum Syrup:
Melt butter in a 2-quart saucepan over low heat; stir in rum, liqueur, and sugar. Bring to a boil over medium-high heat, stirring constantly. Reduce heat to medium; cook 5 minutes or until syrupy. Cool.
After removing the cakes from the pan, poke holes in each cake with a wooden skewer. Brush Rum Syrup over cake. Cool completely on a wire rack.