Cut the brussels sprouts lengthwise in half or quarters depending on their size (if they are small, leave them whole).
Bring a large pot of salted water to a boil. Drop in the brussels sprouts and boil until just tender, 6 to 8 minutes. Drain, shaking to dry them as much as possible.
Place the bacon in a large skillet and cook over medium heat until crispy. With a slotted spoon, transfer the bacon to a paper towel and set aside. There should be about 2 tablespoons of fat in the pan; pour off any excess and discard.
Just before serving, add the butter to the skillet. Increase the heat to medium-high and add the drained sprouts. Cook, tossing and stirring, until heated through and beginning to brown. Stir in the bacon and balsamic vinegar. Season to taste with kosher salt and freshly ground black pepper. Sprinkle on hazelnuts and serve warm.
Notes
If preparing this recipe as GLUTEN FREE, just be sure to use a brand of bacon that is known to be GF.