Butter a square tin and line with greaseproof paper. Set aside.
Place the butter In a largish saucepan over medium heat. Melt the butter until foamy. The foam will eventually clear (be patient) and you will see the clear, golden liquid underneath. Using a wooden spoon stir to make sure the butter isn't burning. It will go a lovely nutty golden brown colour- but do keep your eye on it as it can catch and burn very quickly after this point.
When the butter is a nutty golden brown, add the marshmallows and salt and take the pan off the heat. The residual heat in the pan should melt the marshmallows gently. Stir a little to help the process, but if it ain't happening, then place it back on the heat until the marshmallows are completely melted. Stir until smooth. Add the rice crispies to the marshmallow butter mixture and stir through to coat evenly. Using a spatula, place the mixture into the tin and press down firmly until it's even and fills all the corners. Chill in the fridge for about 2 hours. When cold cut into squares or bars and watch 'em disappear.
Look for big white fluffy marshmallows, not the little ones used for decorating. The American ones called Stay Puff are the best, but having seen them in a very posh deli in London at £4.00 a bag, I think you can get away with using the ones in your local supermarket. Unfortunately I can only find packs of white and pink ones- which give the bars a bit of a girlie hue, but as it's mainly girls around here, I guess that's okay and 'Daddy ' doesn't seem to mind what colour they are as long as we leave him some.