Boil heavy cream and vanilla in a heavy bottomed sauce pan. Let it boil for a minute and lower the heat and simmer.
Simmer the cream until it is reduced to half the amount – it will take about 20 minutes or more depending on how big the pan is and the temperature. Stir occasionally.
Lower the heat and add room temperature butter. Stir into a smooth batter. Remove from heat.
Chop chocolate into small pieces. Add to the warm cream mixture and stir until the chocolate has melted. Add additional flavorings if desired.
Pour into a baking dish, about 7x7 inches (18 x 18 cm), and let cool in the refrigerator for a few hours. Bring out and sprinkle cocoa powder on top. Cut into pieces and serve cold.
This fudge can be kept in the freezer and doesn't need to be thawed. It will be ready to enjoy after just a minute or two at room temperature.
The fudge is delicious as it is, but you can also add a flavor of your choice. Try some instant coffee, licorice powder, peppermint oil or citrus zest. Cinnamon and pumpkin pie spice gives the fudge a holiday flavoring. If you’re bold you can try a sprinkle of smoked chili or sea salt on top. Experiment with the amount; start with 1-2 teaspoons of flavoring.