Sterilize 2 (1 pint) straight sided wide mouth canning jars, lids and rings by boiling for 10 minutes. Keep the lids and rings in the hot water until needed. Preheat oven to 325 degrees F (165 degrees C)
In a small bowl, mix flour, sugar, baking soda and cinnamon. Set aside.
In a medium saucepan, combine butter, water and cocoa powder. Heat and stir until butter is melted and mixture is well blended. Pour into a large bowl.
Stir in Flour mixture and blend. Add buttermilk, egg and vanilla and beat by hand until smooth. Stir in nuts.
Distribute evenly into the 2 prepared canning jars. Place the jars on a cookie sheet and bake at 325 degrees F (165 degrees C) for 35 to 40 minutes or until toothpick inserted deep into center of cake comes out clean.
Using heavy oven mitts, remove cakes from oven one at a time. Place a lid, then a ring onto the jars and screw them down tightly. Place the jars on the counter and listen for them to ping as they seal. If you miss the ping , wait until they are completely cool and press on the top of the lid. If it doesn't move at all, it's sealed.