Heat the oven to 425°F. Spread the bacon pieces on a rimmed baking sheet and bake until the fat renders and the bacon just begins to crisp, 8–10 minutes.
While the bacon cooks, trim the stem ends from the brussels sprouts and remove any damaged outer leaves. Halve the sprouts lengthwise.
Using a slotted spoon, transfer the bacon to a bowl, leaving about 2 tablespoons of bacon fat on the baking sheet. If there is more fat, carefully spoon off the excess; if less, add a little back from the pan drippings.
Add the brussels sprouts to the baking sheet, season with the kosher salt and black pepper, and toss to coat in the bacon fat. Arrange the sprouts cut side down in a single layer.
Roast until the cut sides are deeply browned and the centers are tender, 15–20 minutes, rotating the pan once halfway through.
Sprinkle the bacon over the sprouts and return to the oven to heat and crisp the bacon, 3–5 minutes more.
Transfer to a serving bowl, drizzle with the balsamic vinegar, and toss to coat. Taste and adjust seasoning, then serve hot.