In a small saucepan, combine butter, oil and chocolate. Cook over low heat, stirring until chocolate melts. Add water; cool 15 minutes. In a bowl, combine the remaining cake ingredients and beat well. Fold chocolate mixture into batter.
Pour into two greased and floured 8-in. round baking pans. Bake at 350° for 30-35 minutes or until a toothpick comes outs clean. Cool for 5 minutes before removing from pans to wire racks.
Meanwhile, for filling, combine the milk, sugar and water in a small saucepan. Cook over medium heat until sugar is dissolved. Add raisins and dates. Cook and stir until thickened, about 5 minutes. Remove from the heat; add vanilla and nuts. Cool.
For frosting, melt chips; add vanilla and nuts. Cool. Stir in sour cream and salt; beat with a spoon until smooth. Cool 5 minutes or until frosting achieves spreading consistency.
Place one cake layer on a serving plate. Spread with filling; top with second cake. Frost top and sides of cake. Chill for 1 hour before serving. Garnish with whipped cream if desired.