Combine two cups of brown sugar and one cup water in a small saucepan.
Heat until just boiling, and then reduce heat to medium low and let simmer for five minutes. Stir continuously to keep the sugar from burning.
About two minutes into the simmering, add a tablespoon of Bourbon.
After the mixture has been simmering for five minutes, remove from heat. Add the half stick of butter and stir to combine. I add another teaspoon of Bourbon at this stage for a more complex flavor, but it's optional.
Pour into a glass mason jar for cooling. I use a canning funnel for this part to avoid splashing and burning. If you don't have one, be careful, because the liquid is hot!
Serve immediately. The leftover syrup can be kept refrigerated for up to 30 days. Take care when reheating, so as not to burn the butter. Reheat slowly, and str frequently to recombine the butter with the syrup.