Peel and finely chop the onion. Peel and crush or finely grate the garlic. Halve the chilli, flicking out the seeds and white pith for less heat, and finely chop the chilli.
Pour 1 tbsp oil into a pan and bring to medium heat. Add the onion, garlic and chilli, along with 1 tsp ground cumin and a good pinch of salt and pepper. Fry for 5-6 mins, stirring occasionally, till softened and glossy looking.
Meanwhile, pull the leaves and silks off the sweetcorn cobs. Slice a sliver off the cobs so they stand upright on your chopping board. Run a knife down them to slice off the kernels. Slice the kale leaves off the tough cores and shred the leaves.
When the onion is glossy, stir in the buckwheat, sweetcorn and kale. Crumble in the stock cube and pour in 750ml boiling water. Return to the boil, then pop a lid on the pan. Reduce to a gentle simmer and cook for 10-12 mins, till the buckwheat and sweetcorn are tender.
Taste the soup and add more salt and pepper if needed, then ladle into warm bowls and serve.