Poke holes in the sweet potato and place it on a foil lined baking sheet. Bake for 45minutes ~ 1 hour until fork tender. Remove from the oven and let cool.
When cool enough to handle, halve the sweet potato and scoop out the flesh into a mixing bowl.
Add chickpeas to sweet potato and mash into a coarse paste. Add chili powder, flour, breadcrumbs, parmesan cheese, salt and pepper. Mix until well combined. Divide the mixture into 4 equal size patties.
Heat a large nonstick skillet over medium heat, add 1tbsp oil and 1tbsp butter. Gently place the patties in the skillet and cook until the patties are brown and crisp on both sides, about 5 minutes per side. Serve over onion buns spread with honey mustard sauce and topped with lettuce and avocado.