Uthappam is a type of dosa, which is thicker than the regular dosa with some topping. The topping could be onion, carrot, coconut, tomato, beetroot, cabbage, capsicum, cheese, egg etc.. The topping could be mixture of multiple vegetables as well. Sweet uthappam is made with coconut and sugar topping.
Uthappam is made with the regular idli-dosa batter. Usually whenever I make homemade idli batter, the first 3-4 days I make idli or dosa. After the 4th day, the leftover batter will start becoming sour and may not yield good idlis and dosas. Uthappam is a perfect dish to make with this batter. I often make with fresh batter too because I love uthappams. You can use store bought idli/dosa batter and make this too. The best things about this is that each person gets to choose their favorite topping.