Melt the butter in a medium saucepan over a medium heat and, once foaming, stir in the flour to make a paste. Cook for 1-2 mins, then slowly pour in the milk, whisking continuously until you have a smooth sauce. Bubble for 1-2 mins.

Remove from the heat and stir in the grated cheddar and cheese slices until everything has melted and the sauce is thick – return to the heat for a minute or two if the cheese doesn’t melt completely. Sprinkle in the paprika, then season with salt and pepper.

Warm the sauce over a low heat until piping hot, then pour into a serving dish and sprinkle over the chilli powder, if using. Serve with tortilla chips for dunking.