Render the bacon. Cook the chopped bacon in a large skillet over medium heat for 8 to 10 minutes, stirring occasionally, until crisp. Transfer to a paper towel-lined plate and reserve 2 tablespoons of the rendered fat.

Season. Toss the halved sprouts in a large bowl with the reserved bacon fat, olive oil, kosher salt, and black pepper until evenly coated.

Air fryer method. Preheat to 380°F. Arrange the sprouts cut side down in a single layer, working in batches if needed. Cook 12 to 15 minutes, shaking the basket halfway through, until deeply browned and crisp at the edges.

Oven method. Preheat the oven to 425°F with a sheet pan inside. Arrange the sprouts cut side down on the hot pan and roast 22 to 25 minutes without stirring, until well browned.

Make the honey butter. While the sprouts cook, melt the butter in a small saucepan over low heat. Remove from the heat, then whisk in the honey, apple cider vinegar, salt, and red pepper flakes if using, until smooth.

Toss and serve. Transfer the hot sprouts to a large bowl. Pour the honey butter over, add the reserved bacon, and toss to coat evenly. Taste and adjust salt. Serve immediately.