Hi Marjorie;
This is the recipe I have used for many years.
TEXAS SHEET CAKE
1 1/4 cup margarine or butter
1/2 cup unsweetened cocoa
1 cup water
2 cups unsifted flour
1 1/2 cups firmly packed brown sugar
1 tsp. baking soda
1 tsp. ground cinnamon
1/2 tsp. salt
1 can Eagle Brand condensed milk
2 eggs
1 tsp. vanilla
1 cup confectioners sugar
1 cup chopped nuts.
Prehear oven to 350*. In a small saucepan, melt 1 cup margarine, stir in 1/4 cup cocoa, then water. Bring to a boil; remove from heat. In large mixer bowl, combine flour, brown sugar, baking soda,cinnamon and salt. Add cocoa mixture, beat well. Stir in 1/3 cup Eagle Brand, eggs and vanilla. Pour into greased 15x10" jelly roll pan (cookie sheet with sides)
Bake 15 minutes or until cake springs back when lightly touched.
FROSTING
In small saucepan, melt remaining 1/4 cup margarine; stir in remaining 1/4 cup cocoa and remaining Eagle Brand. Stir in confectioners sugar and nuts. Spread on warm cake.
This is a lot like cake brownies, as you mentioned you were looking for.