In a small saucepan, whisk together the sugar, salt and cocoa powder until well combined. Add the evaporated milk and whisk until blended.
Place the pan over medium heat and add the butter, whisking constantly until butter is melted. Continue whisking until mixture comes to a boil. Remove from heat and stir in the peppermint extract.
Let the sauce cool slightly before spooning over ice cream.
To store, pour the sauce into glass jars and seal. Refrigerate for up to a week. Best when reheated before serving.