Put a large ovenproof cast-iron saute pan, about 25cm wide, on a high flame. Once hot, add the corn kernels and dry-fry for four to five minutes, stirring from time to time, until slightly blackened. Remove from the pan and set aside to cool.
Sift the flour, baking powder, bicarbonate of soda, cumin and cayenne into a large bowl. Add the sugar along with one and a half teaspoons of salt and a good grind of pepper. Stir through and set aside.
Add the polenta, soured cream, eggs and oil to a separate bowl and whisk lightly. Add to the dry ingredients, then fold in the spring onion, coriander, jalapeño and toasted corn until just combined.
Lightly oil the base and sides of the pan used to toast the corn, pour in the cornbread mixture and evenly scatter over all the topping ingredients. Bake for 40-45 minutes, until a skewer comes out clean. Serve the cornbread hot straight from the oven or leave to cool for 30 minutes and serve warm or at room temperature the same day. If serving it the next day, warm through in the oven just before.