Cook the pancetta in a separate pan until brown and crispy. Drain on a paper towel and set aside.
Place Brussels sprouts in the feed tube of the food processor fitted with a large slicing disk (I used the wrong one for these pics as you see in the images below, but it still worked out and tasted fantastic). No need to core them. Process until they are sliced.
Heat the butter and the olive oil in a very large (12-14-inch) sauté pan over medium to medium-high heat. Add the Brussels sprouts, 1/ 1/2 teaspoons of salt and 3/4 teaspoon pepper and sauté, stirring occasionally, for 5 to 7 minutes, until crisp-tender and still bright green.
Remove from the heat and stir in the balsamic vinegar, season to taste and serve hot.