In a large Ziplock bag add the beer, balsamic, salt, garlic powder, and pepper.Add the mushrooms, sealing the bag to remove as much air as possible.Allow to marinate at room temperature for one hour or in the fridge overnight.Remove mushrooms from marinade.
While the mushrooms marinate, make the leeks. Cut the root off and slice lengthwise to expose the inner layers where dirt hides. Wash well under cool water making sure to remove the grit between the leaves. Thinly slice the leeks, discarding the dark green portion.
Heat the oil in a skillet over medium heat. Add the leeks, salt and sugar, cooking until the leeks have softened and are starting to brown. Add the beer. Cook until the leeks have turned a dark golden and the beer is gone. Make sure to cook over medium to medium-low heat, if the heat is too high the leeks will burn before they caramelize.
Heat the oil in a large skillet over medium-high heat until very hot. Add the mushrooms, cooking on each side until seared and softened, about 3 minutes per side (mushrooms can also be cooked on a hot grill with oiled grates).
Add the flesh of the avocados, cilantro, chipotle peppers, salt, and lime juice to a bowl. Mash with a vegetable masher until well combined.
Add the mushrooms to the buns, top with guacamole and tomatoes. Serve immediately.