Preheat the oven to 350 degrees F (175 degrees C). Spray a baking sheet with cooking spray.
Spread out 1/2 cup plus 8 hazelnuts evenly on the prepared baking sheet.
Toast hazelnuts in the preheated oven, stirring frequently, until the skins crack and they turn light brown, 3 to 5 minutes. Be careful not to burn them. Remove from oven and set aside until cool enough to handle. Roll cooled nuts between the palms of your hands to remove the skins.
Set aside 8 hazelnuts and chop them coarsely for garnish. Transfer remaining hazelnuts to a food processor and finely chop.
Melt 1 tablespoon butter in a skillet over medium heat and cook shallots until soft and translucent, about 5 minutes.
Combine remaining 5 tablespoons butter, cooked shallots, and ground hazelnuts in a bowl and blend hazelnut butter well.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add Brussels sprouts, cover, and steam until just tender, about 5 minutes.
Melt hazelnut butter in a pot over low heat. Add Brussels sprouts and toss to coat with the nut butter. Serve sprinkled with reserved chopped hazelnuts.