Let's make potato filling. First take veggies in a pressure cooker, cover with 1 cup of water. Pressure cook for 2 whistle. Let the steam release by itself. Set aside. In a frying pan heat oil, add in fennel seeds and curry leaves. let them sizzle. Add in onions, green chillies and mix well. Add in ginger garlic paste and cook till raw smell leaves. Add in tomatoes cook till mushy. Add in red chilli powder, turmeric powder, garam masala powder, salt and mix well. Add in cooked veggies and mash it using a potato masher. Add coriander leaves and mix well. Potato Filling is ready.
Let's make the bun dough. Take warm water, sugar, yeast in a mixing bowl. Let the yeast activate for 5 mins. It will get foamy. Now add in wheat flour, plain flour, salt and mix well using a spoon. Add in soft butter and knead it well. Knead it for 5 minutes till the dough is smooth. Cover the dough with a cloth and let it rest for 1 hour till it doubles in size.
Transfer the dough into a silicon mat or work surface. Divide the dough into 8 equal balls and set it aside. Take the dough ball and flatten it slightly, place the filling inside this and seal it from all edges. Make it into a round shape and place on a foil lined baking sheet. Make every dough like this and place it in a baking sheet leaving some space between them, because they will expand.
Cover it with a cloth and let it rise for 30 to 45 minutes. If you want you could brush the buns with a beaten egg or milk. Sprinkle top with sesame seeds.
Now place this in a 200 degree C preheated oven for 20 minutes till lightly golden on the top. Remove it from the oven and brush it with some butter and serve warm.