Using an emulsion blender, mix the butter with the sage. Set aside.
Prepare your grill for direct and indirect cooking zones. Place your cast iron skillet on indirect heat but very close to the direct cooking zone. If using your stove top, place your skillet on medium/medium high heat.
Add 1/2 of the butter (1/4 cup) to the skillet. Add the Brussels sprouts and let them get coated in the butter.
Stir the sprouts frequently so they don't stick to the pan, and add the rice wing vinegar.
Next, add the chopped up prosciutto. Let the sprouts cook for about 15 to 20 minutes or until the liquid has cooked down and the sprouts are cooked all the way through. They will be bright green in color.
Serve with full confidence that your family will LOVE these Brussels sprouts.