Soak the corn husks in plenty of warm water for 10 minutes until pliable, then drain.
Heat the oil in a frying pan, add the onion and cook for 5 minutes until softened. Add the chard stalks and cook for 3 minutes until softened. Add the chard leaves, garlic, chilli and tomato and cook for a further 1 minute. Stir in the crème fraiche and leave to bubble until it just coats the vegetables. Leave to cool.
Meanwhile, beat together the masa harina, baking powder, salt and lard in a bowl, then beat in the stock until a soft dough forms.
Lay out 1 corn husk so it is flat, place a spoonful of the dough in the centre and down towards the bottom of the husk, then flatten with your hand. Place a spoonful of the chard mixture on top, then fold over the husk so the filling is tightly enclosed and forms a cylinder. Leave the top end open and secure with strips of leftover husk. Repeat with the remaining ingredients.
Place the parcels, sealed end down, into a steamer. Cover and steam for about 1 hour, until the dough comes away easily from the husk.