Place dried cherries in a bowl.
Warm the cognac in a saucepan on low heat.
Pour warm cognac over the dried cherries. Allow the cherries to soak for a minimum of 8 hours, but preferably up to 24 hours.
Place the reconstituted cherries with any remaining cognac in a food processor and process until smooth.
Bring the demi glace to a boil and reduce by half.
Add the processed cherry mixture, heavy cream, salt and pepper. Bring up to a simmer; remove from heat.
Whisk the cold butter into the mixture.