Add a few layers of paper towels to a plate and set aside.
Heat a large skillet over medium heat and, once slightly smoking, add the bacon to the pan.
Saute and cook until the bacon is crisp and golden brown. About 3-4 minutes.
Remove the bacon with a slotted spoon and place on the paper towel lined plate.
Pour off all but 1 tbsp of the bacon grease, and place the Brussels sprouts, cut side down, in a single layer in the pan.
Cook the Brussels sprouts until they are golden brown and the sugars have caramelized, about 3-5 minutes. Add salt and pepper, 1/3 of the apple cider, and toss and saute until the apple cider has reduced and almost cooked off.
Add another 1/3 of the apple cider and toss and saute, as before, until the apple cider has reduced and almost cooked off.
Add the last 1/3 of the cider and toss and saute until the cider has reduced and the sprouts have completely cooked through.
Add more salt and pepper (if needed), toss the bacon bits back in then add the butter and stir until melted.