Cake: Melt margarine and cocoa. Let cool. Add eggs, one at a time, beating after each addition. Add flour and vanilla flavoring, mixing well. Pour into a greased and floured 9 x 13 pan or 12 x 18 jelly roll pan. Bake 9 x 13 approximately 30 minutes or 12 x 18 approximately 20 minutes at 350 degrees. Do not overbake. Remove from oven and while still hot, sprinkle marshmallows and chocolate chips over cake. Return to oven for 5-8 minutes to melt marshmallows and chocolate chips (will not melt completely}. Remove from oven and immediately pour Chocolate Frosting over cake.
Chocolate Frosting: Melt margarine and cocoa. Add evaporated milk, stirring until smooth. Bring to a boil, stirring all the while. Boil 1 minute. Add sifted powdered sugar and vanilla flavoring, mixing well. If too thick, add a little more milk. Pour over top of marshmallows and chips.