Combine together flour, cornflour and chili powder in a large bowl. Add water little by little to make thick batter without any lumps. Season with salt and pepper. Set aside.
Cornflakes coating:
Place cornflakes in a shallow plate. Season with salt and pepper. Set aside.
Fry paneer:
Dip the paneer block first in the batter and then coat in cornflakes.
Meanwhile, heat oil over medium-high heat. Place paneer in hot oil. Fry both sides until golden. Drain on an absorbent towel. Set aside.
Spicy mayo:
Combine together mayonnaise, hot chili sauce and lemon juice in a small bowl. Set aside.
Assemble sandwich:
Spread mayonnaise over one bread slice. Top with lettuce and paneer. Place the other bread slice on top. Carefully tip it over so that the bread slice with mayo comes on top. Serve warm.