Combine 1 1/2 cup butter and water in a medium saucepan. Bring to a boil over medium heat, stirring occasionally. Remove from heat, and add 4 ounces white chocolate. Stir until chocolate melts. Stir in buttermilk, eggs, and 1 1/2 teaspoons vanilla. Gradually stir in flour mixture; batter will be thin. Dredge pecans in 1/2 cup flour, and fold into batter. Pour into prepared pans.