Preheat oven to 350º. Grease and lightly flour two 8x8x2” baking pans.
Sift flour and soda into a large bowl. Set aside.
In a small saucepan, combine butter, oil and chocolate; stir over low heat to melt the chocolate. Add 1 cup water. Cool 15 min.
Add 2 cups sugar, eggs, sour milk and vanilla to flour, mix well. Stir in cooled chocolate to combine. Spoon batter into prepared pans, dividing equally between pans. Bake 30-35 min or until cake springs bake when pressed with finger. Cool in pans 5 min. Turn out on wire racks to finish cooling.
Melt 12 ounces of chocolate chips in top of double boiler. Remove top of double boiler from hot water and stir in sour cream and a dash of salt. Beat until smooth. Cool until frosting is of spreading consistency.
Place one cake on a serving platter top side down and with a spatula cover to edges with 1/3 of frosting. Set second cake on top of filling, top side up, and frost sides and top with remaining frosting.