Salted caramel is a sticky filling that binds layers and adds a bitter-salty contrast. The common problems are sugar crystals and caramel that sets like stone. The first comes from impurities and too much stirring, the second from cooking too long.

Ingredients and what each one does

Sugar is the base. As it heats it breaks down into aromatic compounds, which is where the caramel flavour comes from.
Cream loosens the caramel and makes it soft. Cold cream hitting hot sugar boils violently and seizes into lumps, so it is warmed first.
Butter adds smoothness and gloss.
Salt, a pinch, sharpens the flavour. Without it the caramel is just sweet.

How to make it

Prepare everything first: cream warm (about 40 to 60 °C), butter cubed, salt at hand. There is no time later.
Spread the sugar in an even layer in a dry heavy-bottomed pan over medium heat. Do not stir: crystals on the pan wall can start crystallisation of the whole batch. Swirl the pan if needed.
When the sugar melts, stir gently. Cook to a deep amber, roughly 170 to 180 °C. Sign: the smell turns from sweet to caramel. Darker turns bitter, lighter tastes flat.
Off the heat, pour in the cream in a thin stream, carefully: it foams and spits. Stir until smooth.
Add the butter and salt and stir. Return to the heat for a minute or two if it is too thin.
Cool it. The true thickness only shows when cold.

What can go wrong

Crystals. Grains from the pan wall, impurities or stirring too early. Add a splash of water and reheat until dissolved.
Sets like stone. Cooked too long, or too little cream.
Too thin. Not cooked to the right colour, or too much cream.
Bitter. The sugar was overheated.

Substitutions

A spoonful of glucose syrup lowers the risk of crystallisation. Milk instead of cream gives a thinner, less rich caramel.

Storage

In a closed jar in the fridge for up to 2 weeks. Warm it slightly before using.