Mix sugar, butter, milk and bourbon together until smooth, let simmer for 5-7 minutes over medium-heat, whisking constantly.
Remove from heat and let stand to thicken.
Using a piping bag or a warm knife, drizzle slowly over Cinnamon Sugar Cookies, slather on cake or just eat straight out of the jar. Sprinkle with sea salt while still hot.
Notes
Adapted from http://petitechouette.com/
Recipe by She Eats at https://sheeats.ca/2014/03/boozy-dulce-de-leche/