Prepare either 16 4-ounce or 8 8-ounce mason jars, laying them out on a cookie sheet with lids off. This will make it easier to move into the water bath and provide a cooling/storage area when you are finished.
Heat the cream in a pan over medium-high heat until the cream around the edges of the pan just starts to bubble. Do not let the cream come to a full boil. Remove from heat.
While the cream is heating, break the chocolate bars into pieces. When the cream is hot, whisk in the chocolate, 2.5 teaspoons espresso powder, 6 tablespoons sugar, kosher salt and bourbon. Whisk until smooth.
Whisk in egg yolks one at a time, whisking completely after each addition. Pour the mixture into a large measuring cup suitable for pouring.
Pour mixture into the mason jars, leaving some space at the top of each jar. Do not fill to the top.
Screw the lids on until they are only finger tight. Do not overly tighten as air needs to escape from the jars during the cooking process.
Fill a large pot or a 12-quart plastic storage container with room temperature water and secure your sous vide circulator. Place the jars in the pot or container. Turn the circulator on and set it to 176°F (80°C). When the water reaches 176 degrees set the timer for one hour. Putting the jars in the water while it's at room temperature will keep them from cracking during the cooking process.
While the jars are cooking, whisk together 1/2 cup of crème fraiche, 1 teaspoon sugar and 1/2 teaspoon espresso powder. Whisk until thoroughly combined, cover and store in the fridge.
When the timer goes off, line the cookie sheet with paper towels or a kitchen towel and remove the mason jars to the tray with a pair of tongs. Let them cool for 30 minutes, then refrigerate for at least two hours. These will keep in the fridge up to a week, but they won't last that long!
When ready to serve, remove the lids, sprinkle with sea salt and add a generous spoonful of the creme fraiche mixture to the top of each jar. If you like, garnish with a raspberry or chocolate covered espresso bean.
