3 1/2 ounces dark chocolate (such as Green & Black’s Organic 70% Dark Chocolate), broken into pieces
1 cup heavy cream
1 teaspoon vanilla extract
1 (14 ounce) can condensed milk
1 1/4 cups Scotch whiskey
1 (1.25 ounce) shot brewed espresso, cooled to room temperature
Melt chocolate with cream in a double boiler over medium-low heat, stirring constantly until the mixture is smooth; remove from heat and set aside to cool.
Whisk the eggs and vanilla extract together in a large bowl until combined; stir the condensed milk into the mixture. Slowly pour the whiskey into the egg mixture while stirring until completely incorporated. Add the chocolate mixture and the espresso; beat with the whisk until the mixture is uniform in color.
Pour into a bottle, cap, and chill in refrigerator at least 4 hours before enjoying. Store in the refrigerator.