When it calls for ½ tsp dried chilli in the Chicken Mughlai - is that chili powder?
Nigella's Mughlai Chicken (from FEAST) is an aromatic korma-style curry that is made with a spice base that includes dried chillies. The chillies referred to in the recipe are dried chilli flakes, also known as crushed red pepper flakes. You should be able to find them in most supermarkets.
Most chili powders are a combination of powdered chillies, cumin, coriander and sometimes other spices, as they are usually used for dishes like chilli con carne rather than for curries. So we would generally avoid using them for this dish. However, if you have pure, powdered chillies then you may need to reduce the amount used in the recipe. We would at least halve the amount to ¼ teaspoon and maybe even less if it is a very hot powder. You are aiming for spicy warmth rather than mouth-searing heat.