First of all, cook the potatoes. You can use any method you like but I find the best way is to microwave them as this is not just quick and easy but also dries them out a bit and that is only a good thing when you're aiming for crispness. Wash and drain the potatoes, then place in a large microwave-proof bowl. Microwave on high for 5 minutes. Check for doneness. Take out any small ones which might have already been fully cooked. Turn the remaining potatoes over and microwave in bursts of 5 minutes and repeating the process of taking out any that's cooked. Larger potatoes will probably take about 20 minutes in total. Set aside to cool.
Process the bread into fine crumbs. You can trim the crusts if you want to, but I don't. Gently melt the butter, if using.
Peel the potatoes and mash them in the same large bowl. Or if they are not easily mashed, grate them coarsely.
Add the breadcrumbs, the melted butter (if using) and all the other ingredients. Mash well together, forming a homogenous mixture. Taking about a scant handful (approximately 1/8 - 1/4 cup) of the mixture at a time, roll them into smooth crack-free balls and then press down slightly to form a disc. The flatter and thinner this disc, the more surface area for greater crispness.
Heat a frying pan (cast iron for preference: see tip above) over medium heat and add a tbsp of oil in the centre. Add 5-6 tikkis to the oil, let them sizzle a bit in the oil then flip and continue with the other side. Once slightly golden on either side, move to the periphery of the pan and let them cook on the lower heat while you repeat this process of frying tikkis in the centre and moving them to the side. Add oil as necessary. You don't need a lot, just enough.
Keep flipping and checking the tikkis on the periphery until they are crisp. Remove to a plate as each looks done.
Serve on their own with ketchup, chilli sauce or chutney. Or to make a meal out of them, top with chole (chickpea curry), green and/or brown chutneys, chopped onions, tomatoes, green chillies and coriander and sev.