Peel back the husks of the sweetcorn, leaving them attached to the cobs. Remove the silky bits and wash the cobs well.
Mix the chaat masala and chilli powder in a small bowl. Preheat the barbecue. Grill the sweetcorn directly over a high heat, turning often with the leaves, until lightly charred all over (if it rains you can do this directly over a gas flame). When cooked, smear each cob with butter or crème fraîche, if you want.
Serve the sweetcorn with the lemon wedges and a little mound of the spice blend on the side. Dip the wedge in the spices and rub them over the sweetcorn, squeezing a little lemon juice on as you do and serve.