Chop up the white chocolate baking bars (8 oz total), reserving half for the frosting.
In a large saucepan, bring margarine or butter and 3/4 cup water to boiling, stirring constantly. Remove from heat. Add baking bar, stirring until melted. Stir in buttermilk, eggs and rum extract and set aside.
Stir together 1/2 cup of the flour and the pecans and set aside.
In an extra-large bowl, stir together the remaining 3 cups flour, sugar, coconut, soda and baking powder. Stir-in the margarine/butter mixture. Fold in the pecan mixture.
Pour into 3 greased and floured 9 x 1 1/2-inch round baking pans. Bake in a 350 degree oven for 25 to 30 minutes or until done.
Cool in pans on wire racks for 10 minutes. Remove from pans and complete the cooling on the wire racks.
To prepare the frosting, melt the reserved 4 oz of white chocolate bar. Cool 10 minutes. In a large mixing bowl, beat 1/2 cup softened margarine or butter, and the softened cream cheese (note the 11 oz is one 8-oz and one 3-oz packages), with an electric mixer until combined. Beat in the chocolate. Gradually add the confectioners sugar, beating until smooth.
To assemble, place a cake layer on a platter. Spread with 1/2 cup of the frosting. Repeat layers. Top with remaining cake layer. Frost top and sides with remaining frosting. Garnish by sprinkling the toasted coconut in a large circle on top of the cake.