Bring to boil margarine, milk and cocoa. Add powdered sugar and vanilla; mix well.
Cool completely before spreading on cake.
As the frosting cools it will thicken up which is good since it is thin after cooking it on the stove-top. (I cooled frosting for about 20-30 minutes in the refrigerator stirring a few times to make sure it was thickening up)
Once cooled then spread on the cake brownies. Top with nuts. The nuts can be optional.
Recipe by JAQUO Lifestyle Magazine at https://jaquo.com/sheet-cake-brownies/