Heat first 5 ingredients together in medium saucepan until bubbly.
In a mixing bowl, mix together the flour, baking soda, and sugar. Add the bubbly cocoa mixture and vanilla. Beat until smooth.
Pour into greased and floured 9 x 12 cake pan and bake for 30 minutes in 350 degree preheated oven. Remove from oven and cool completely.
Spread peanut butter over cooled cake.
Heat together the 1 stick of margarine, 4 Tbls. cocoa, and 6 Tbls. buttermilk until bubbly. Remove from heat and pour over powdered sugar. Add vanilla and beat until smooth. Pour over peanut butter layer. Allow to cool completely.