Place the sugar in a large saucepan over medium heat.
Let the sugar melt slowly. From time to time, lift the pan over the heat and use a spatula to move it around without stirring the inside.
When the sugar has almost completely melted, stir with a wooden spoon to melt the stubborn pieces. You can lift the saucepan above the heat and stir well, if you see that the coloring is going too fast.
When the caramel is close to the desired color, heat the heavy cream.
Once the sugar has taken on a nice brown color and has completely melted, turn off the heat or remove the pan from the heat and pour the hot heavy cream over it in two stages. Stir constantly with a whisk.
Turn the heat back on or put the pan back on the stove. While mixing, add the butter.
Cook for 3 minutes to thicken slightly. If you have a thermometer, remove from the heat at 105°C (221°F).
The caramel will thicken as it cools.
Leave to cool slightly, stirring occasionally, before potting. Allow to cool completely before sealing the jar.