Bring to boil, stirring continuously. Cook the sugar mixture till it starts bubbling.
Add soft butter and mix it in the sugar mixture. Bring it to boil. Add salt in this step when using unsalted butter.
½ cup Butter
Remove the pan from heat let it cool it for a minute. Keep stirring not to further cook the sugar.
Add cream and keep stirring the mixture. Add it slowly to the mixture, as hot mixture can splatter when adding the cold cream.
½ tsp Salt
Return the pan to heat and stir continuously for next 2 minutes or till the mixture reaches 240°F or 115°C. Check if you have a food thermometer. Or, simply when the mixture starts to thicken.
Let it cool a bit and transfer to a heat-proof bowl and microwave safe bowl.