Aloo gobi tawa burgers

Name: Aloo gobi tawa burgers
Ingredients: 3 large baking potatoes (about 1kg/2lb 4oz); 2 tbsp neutral oil, such as rapeseed oil; 1 medium-sized cauliflower (about 500g/1lb 2oz), stem removed and finely chopped; ½ tsp nigella seeds; 2 thin green chillies, finely chopped; 1 tbsp grated ginger; 1 tsp coriander seeds, coarsley ground; ½ tsp turmeric; 1 tsp salt; 1 large tomato, finely chopped; 1 tbsp garam masala; 2 tbsp fresh coriander, finely chopped; 6 large sesame burger buns, sliced in half; 4 tbsp tamarind chutney, shop bought; 3 tbsp butter or oil; 1 tsp Kashmiri chilli powder or sweet smoked paprika