Stir together first 5 ingredients. Fill 1 (16-oz.) glass or Mason jar with crushed ice. Pour bourbon mixture over ice; top with ginger beer (about 1/3 cup). Garnish, if desired. Serve immediately.
Note: We tested with Maker's Mark Bourbon, Domaine de Canton ginger liqueur, and Reed's Extra Ginger Brew and Goya Jamaican Style Ginger Beer.