1. boil 2 large potatoes, about 350 grams till they are completely cooked. peel and then mash them with a fork in a bowl.
2. heat 2 to 3 tsp oil in a small pan. add 1/2 tsp mustard seeds and crackle them. then add 7 to 8 curry leaves and asafoetida. stir and saute for about 5 seconds.
3. then add 6 to 7 garlic and 1 to 2 green chilies, which have been crushed in a mortar-pestle. add 1/8 tsp turmeric powder. stir till the raw aroma of garlic goes away.
4. pour this tempering in the mashed potatoes.
5. add 1 to 2 tbsp chopped coriander leaves. also add salt.
6. mix everything well and then make small to medium balls from the mashed potato mixture. flatten these balls a bit. cover and keep aside.
7. in another bowl, make a smooth batter from 1 to 1.25 cups besan/gram flour, 1/8 tsp turmeric powder, a pinch of asafoetida, a pinch of baking soda (optional), salt and 1/2 cup water. the batter should not be too thick or thin. if the batter becomes thin, add 1 or 2 tbsp of gram flour. if the batter becomes thick, then add 1 or 2 tbsp water. keep aside.
8. in a kadai or pan, heat oil for deep frying.
9. dip the slightly flattened potato balls in the batter and coat it evenly with the batter.
10. gently drop these batter coated potato balls in medium hot oil.
11. depending on the size of the kadai or pan, you can add more or less of the vadas while frying.
12. deep fry these batatavadas till golden evenly.
13. when they are still hot or warm, you need to serve them.
15.place the hot batatavada sandwiched in the bread slices.
16.serve vadapav immediately or else the pavbecomes soggy.