Wash and chop the rhubarb stalks into about 1/2-inch pieces. Add the rhubarb, sugar and lemon juice to a small sauce pan and cook (uncovered) over medium-high heat, stirring often, until the mixture comes to a gentle boil. Turn the heat down and simmer until rhubarb is completely broken down and mixture has thickened, about 30-40 minutes.

Allow to cool completely and store in an air-tight container in the refrigerator for up to two weeks. Freeze the butter to keep longer.

In each copper mug, mix together 2 oz. of vodka with 1 tablespoon of the rhubarb butter. Add lots of ice to each mug and pour one Ginger Beer into each mug. Mix and serve. Garnish with a stalk of fresh rhubarb if desired.