Cook the bacon in a large oven-safe skillet over medium heat until crisp, about 7 minutes. Transfer the bacon to a paper-towel-lined plate, leaving the fat in the skillet.
Add the butter to the bacon fat and let it melt, then continue cooking to brown the butter, swirling the pan, until it turns golden brown and smells nutty, about 3 minutes.
Add the brussels sprouts to the skillet, cut side down, and season with the salt and pepper. Cook undisturbed for 4 to 5 minutes, until the cut sides are deeply caramelized.
At this point, you can stop and make ahead: let the sprouts cool, then refrigerate in an airtight container up to 2 days. When ready to serve, spread on a sheet pan and roast at 425°F for 12 to 15 minutes, until heated through and re-crisped, then continue with the glaze below.
If serving right away: transfer the skillet to a 425°F oven and roast for 15 to 18 minutes, until fork-tender and deeply caramelized.
Whisk together the maple syrup and apple cider vinegar. Toss the hot brussels sprouts with the maple mixture, the reserved bacon, and toasted pecans. Taste and adjust salt.
Finish with flaky salt right before serving.