Preheat oven to 400 F.
Cook bacon to desired doneness – I like to stop just short of crispy, for this recipe.
As bacon is cooking, remove any yellowed or otherwise ugly outer leaves from the Brussels sprouts. Cut off the very end (brown) of each stem end, before cutting each sprout in half. Place on a baking sheet.
Use a slotted spoon to transfer bacon from pan, setting aside. Measure 1/4 cup of bacon drippings into a bowl or measuring glass, set aside for now.
Pour remaining bacon drippings over the Brussels sprouts, tossing to coat well. Sprinkle sprouts with brown sugar, season with salt and pepper, gentle toss to coat. Roast for about 25 minutes, stirring the sprouts every 8-10 minutes or so.
Whisk cider vinegar, mustard, garlic, 1/2 tsp pepper, and 1/4 tsp salt into the the (slightly cooled!) reserved 1/4 cup of bacon drippings until mixture becomes thick, well combined, and smooth.
Once sprouts are as roasted as you would like them, remove from the oven and transfer to a serving bowl. Toss with vinaigrette and bacon, season with more salt and pepper to taste.