This technique for making salted caramel sauce is called the "dry" method because it doesn't use any water. Instead, you use high heat to melt the sugar directly without dissolving it first. It produces a very rich intense caramelized taste (with slight burnt sugar bitterness) and a very dark amber color.
First warm up the cream. Place the cream in a heatproof measuring jug and warm it in the microwave so it is warm to the touch. It doesn't need to boil.
Pour sugar in an even layer in a 1- or 2-quart stainless steel saucepan (preferably with a wide base). Place the pan over medium-high heat and let the sugar melt.
As you see the sugar crystals around the sides of the pan start melt, turn golden and liquefy, use a wooden spoon or heat resistant silicone spatula to push sugar from the edges to the centre of the pan and then push the unmelted sugar from the centre out to the sides to help it melt evenly. Continue to move the sugar like this until it all begins to turn golden and liquefy. It will clump up and look dry before it liquefies, but trust that it will once it all gets uniformly hot enough to melt entirely.
Continue to move the sugar gently until it all melts and transforms into a clear amber syrup and use the wooden spoon to press out any large lumps of sugar. Reduce the heat slightly if necessary to control the cooking process and prevent the syrup from getting too dark before all of the sugar lumps melt completely. Do not add the butter until there are absolutely no lumps of sugar or else the caramel will turn grainy once it cools.
Once it is copper-colored and lump-free, remove from heat and carefully whisk in butter. The mixture will bubble up like lava, so be cautious.
Slowly pour in the warm cream, then whisk until smooth. Mix in salt. If you still feel some hard bits of sugar along the base of the pan, return the pan to medium heat and cook while stirring just until melted. Stir in vanilla if you choose to use it. Pour the sauce into a heatproof jar and let cool at room temperature before storing in the fridge.