Grease two 1-pint straight-sided wide-mouth canning jars; line the bottom of each jar with waxed paper. Set aside. In a small bowl sitr together flour, sugar, baking soda and cinnamon, if desired. S et aside. In a med. saucepan combine margarine, water, and cocoa powder; heat an d stir till margarine is melted and mixture is well blended. Remove from heat; stir in flour mixture. Add buttermilk, egg and vanilla; beat by hand till smooth. Stir in nuts. Pour mixture into the prepared canning jars. Cover the jar s tightly with greased foil. (Place greased-side down on each jar). Place jars in a 3 1/2, 4, 5, or 6-quart crockery cooker with liner in place. Cover; cook o n high heat setting for 2 3/4 to 3 hours or till cakes spring back when touched and a long wooden to oth-pick inserted near the centers comes out clean. Remove jars from cooker; cool 10 minutes. Unmold cakes; remove waxed paper. Serve warm or cool with ice cream. Makes 8 servings. * WARNING: Use only CANNING JARS for this recipe. Others may not be tempered to withstand the heat. Do NOT use coffee or vegetable cans as most contain lead and are painted o r sealed with materials that may give off toxic gases when heated.