1. Prepare a grill for medium heat.
2. Split the butternut squash and courgette lengthwise down the middle. Brush them and the corn with olive oil, sprinkle with salt and pepper and grill until the veggies are somewhat soft and have nice grill marks. Allow to cool slightly, then cut the squash and courgette into large dice. Scrape the corn from the cob.
3. Warm the tortillas in a microwave. Heat the black beans in a pot and mash with a fork.
4. To build the burritos, heap some rice, mashed beans and diced veggies in the middle of each tortilla. Top with grated cheese, chopped tomatoes, chopped onions and sour cream. Tuck in the ends and roll into a burrito shape.