Take a muslin cloth and put the yogurt in it. Tie the cloth into a knot, and hang it in the fridge with a bowl beneath it to catch the whey that drips out.
Leave it in the fridge for 3 hours or overnight until all the whey has dripped out, leaving you with thick and creamy yogurt.
In a bowl, combine the hung yogurt, black olives, green bell pepper, carrot, cabbage, corn, green chili, salt, and ground black pepper. Mix everything together with a spoon, and set it aside.
Take a slice of bread and spread 2 to 3 tablespoons of the hung yogurt mixture evenly on it. Place another slice of bread on top to make a sandwich.
Spread about a teaspoon of softened butter on both sides of the sandwich, ensuring even coverage.
Heat a saucepan over high heat. Once it's hot, place the sandwich in the middle and reduce the heat to medium. Toast the sandwich on both sides for 2-3 minutes or until it turns golden brown.
After toasting, slice the sandwich diagonally, and serve it hot!