Preheat the oven to 220°C/gas mark 7. Combine the chilli powder, cumin, paprika, chillies and oregano for your spice mix. Drizzle the sweet potato and cauliflower with the oil, then toss in the spice mix. Spread out in a single layer over one or two baking trays. Roast for 20 minutes, then turn and roast for a further 15-20 minutes, until the vegetables start to caramelise and crisp up.
Meanwhile, heat a griddle pan over a medium-high heat. Add the corn and cook for 18-20 minutes, turning occasionally, until tender and just charred. Wrap in foil and set aside to cool.
Transfer the roasted vegetables to an ovenproof serving dish and empty half the black beans around the middle. Sprinkle over the cheese, then return to the oven for 5 minutes.
Meanwhile, mash together the avocado and yogurt with 2 tsp lime juice and season well. Carefully cut the corn kernels from the cob and toss with the remaining black beans, jalapeño, spring onions, coriander and the juice of half a lime. Season.
Once the cheese has melted, remove the vegetables and beans from the oven and leave to rest for a few minutes. Cover with the salsa and spoon over the guacamole. Serve with some lime wedges.