Make sure you have everything ready to go, the caramel is a fast process and the sugar will burn quickly.
Heat the sugar on medium-high in a 2-3 quart heavy bottomed saucepan. Stir the sugar constantly with a whisk as it melts.
As soon as all of the sugar crystals have melted (the liquid will be a dark amber color), immediately add all of the butter and whisk vigorously until the butter is melted.
Take the pan off of the heat as soon as the butter is melted and add the cream while continually whisking to mix. The mixture will foam up when the cream is added, which is why it is imperative that you remove the pan from heat beforehand.
Whisk until the caramel is smooth and allow it to cool for at least five minutes. The mixture will be very hot.
Once the mixture is slightly cooled, whisk in the chocolate pieces and stir until blended completely. Eliminate any chunks or still-in-tact chocolate chips.
Slowly whisk in the condensed milk and allow the mixture to cool for another three to five minutes before serving or storage.
Makes great fondue, ice cream topping, or spread when refrigerated over night. Recipe makes about three cups of sauce, so store accordingly and use quickly. Reheat sauce after refrigeration to thin back to original consistency. Enjoy!