Put yeast and castor sugar in a bowl and mix. Add two tablespoons of warm water, mix well and set aside to activate.
Heat half tablespoon oil in a non-stick pan, add mustard seeds and let them splutter.
Add green chillies, curry leaves, turmeric powder, coriander powder and two tablespoons water and mix well. Add potatoes and toss well.
Add coriander leaves, salt and lemon juice and mix well. Remove from heat and set aside to cool.
Place flour on the worktop and make a well in the center. Add milk powder, salt and activated yeast mixture in the well, mix and knead into a soft dough using water as required.
Add two and a half tablespoons butter and knead again. Cover with a damp muslin cloth and set aside to ferment for twenty to twenty-five minutes.
Preheat oven to 180° C. Grease a cake tin with some oil.
Dust the worktop with some flour, place the dough on it and roll. Divide the dough into eight equal portions shaped into balls. Make a cavity in the centre ensuring that the edges are thin, while the center is thick. Stuff cooked potato mixture in the center and top it up with vada pav chutney. Shape into smooth balls.
Place the stuffed vada pavs in the greased tin, cover with a damp muslin cloth and set aside to prove for fifteen minutes.
Brush the proved vada pavs with some milk. Place the tin in the preheated oven and bake for fifteen to twenty minutes.
Remove from oven, brush with the melted butter, remove from the tin and serve hot.