Whisk brown sugar and Guinness together until frothy. Line a baking sheet with aluminum foil, place baking rack on top, and line rack with bacon. Bake for 10 minutes, then flip bacon and generously brush with Guinness and brown sugar mixture. Bake another 10 minutes then flip bacon again and brush with Guinness and brown sugar. Bake another 10 minutes and baste again. Continue until desired crispiness.