First off, you will need to shave the Brussels sprouts. The most efficient way to get this done is by using the slicing disk of your food processor, but alternatively, you could grate them with a box grater or slice them thinly with a knife.
Then add the mayo, mustard, garlic, chipotle powder, vinegar, salt, pepper, onion powder and chili flakes to a small food processor and process until smooth and creamy. Pour this dressing over the shaved Brussels sprouts and mix really well, ideally with your hands, massaging and bruising the sprouts a little bit to get them to soften up and release some of their water. Set aside.
In a skillet set over medium heat, cook the bacon until nice and crispy, then throw the hazelnuts right in. Continue cooking for about 30 seconds, then immediately pour all of this, including the bacon fat, right into the reserved salad.