Cook bacon in a large cast iron skillet over medium heat until just crisp, 5 to 7 minutes. Transfer to a paper towel-lined plate, reserving bacon grease in the skillet.

Place Brussels sprouts in the skillet with the cut-sides down. Increase heat to medium-high and saute in the bacon grease until lightly browned, 2 to 3 minutes.

Add garlic and saute until fragrant, about 30 seconds.

Pour in chicken stock and cover skillet with a lid; simmer until Brussels sprouts are bright green, 3 to 5 minutes. Remove the lid and continue simmering, until liquid is evaporated and sprouts are at desired tenderness, 3 to 5 more minutes.

Remove skillet from the heat. Add bacon, butter, balsamic vinegar, salt, and pepper; stir until butter is melted.