Sift the flour, baking powder and a pinch of salt into a bowl and make a well in the centre. Add the eggs and milk and gradually beat them in, using a balloon whisk or a wooden spoon, drawing in the flour from the sides to make a thick, smooth batter. Stir in the corn and, if you are using it, the chilli, then season with pepper. Set aside until ready to cook.
To make the dip, stir the chopped watercress and lemon juice into the yoghurt. Cover and refrigerate until serving.
To cook the fritters, lightly oil a large, heavy-based frying pan and heat over a moderate heat. Drop large spoonfuls of the batter into the pan, making a few fritters at a time, and cook for 2 minutes until browned on the bottom.
Using a spatula, turn the fritters over and cook for a further 1–2 minutes, or until the second side is cooked. Remove and drain on paper towels. Keep warm in a low oven while cooking the rest of the fritters in the same way, lightly oiling the pan between batches. Serve with the dip.