Beat 3/4 cup butter at medium speed of an electric mixer until creamy. Gradually add 1 1/2 cups sugar, beating well. Add eggs, one at a time, beating after each addition. Combine flour, baking powder, and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in 2 teaspoons vanilla.
Pour batter into a greased and floured 13- x 9- x 2-inch pan. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Remove from oven. Punch holes in cake with wooden pick.
Combine pineapple juice, cream of coconut, and rum; stir well. Drizzle juice mixture over warm cake. Cool completely in pan on a wire rack.
Beat remaining 1 cup butter at medium speed of electric mixer until creamy; gradually add powdered sugar, beating well. Add preserves and remaining 1/2 teaspoon vanilla, beating at medium speed of electric mixer until of spreading consistency. Spread frosting on top of cake; sprinkle with coconut and macadamia nuts.