Preheat oven to 350°F Sift flour with soda into large bowl. Grease well and flour two 8x8x2" square cake pans. In small saucepan, combine butter, oil and chocolate; stir over low heat to melt the chocolate. Add 1 cup water. Cool 15 minute.
To flour mixture, add 2 cups sugar, the eggs, sour milk and 1 teaspoons vanilla; mix with wooden spoon. Stir in cooled chocolate just to combine. Quickly turn into prepared pans; bake 30-35 minute til surface springs back when pressed with finger.
Cool in pans 5 minute Carefully loosen sides with spatula. Turn out on racks; cool.
Make filling: In small saucepan, combine milk, sugar and 1/4 cup water. Cook over medium heat, stirring to dissolve sugar. Add raisins and dates. Stir with wooden spoon. Cook, stirring, until mixture is thickened, about 5 minute Add vanilla and nuts. Cool completely in small bowl; beat cream with rotary beater just until stiff. On plate, place layer, top side down; spread with filling, then whipped cream. Top with second layer.
Make frosting: Melt chocolate pieces in top of double boiler over hot water. Remove top of double boiler from hot water. Stir in sour cream and salt. With wooden spoon, beat until smooth. Cool 5 min., until frosting is of spreading consistency.
With spatula, frost top of cake, swirling decoratively; use rest of frosting to cover the sides. Refrigerate one hour before serving. To serve: with sharp, thin knife, mark top of cake into four quarters; then cut each quarter into four slices. Makes 16 servings.