Line an 8-inch square glass baking dish with aluminum foil, overlapping sides.
Combine bittersweet chocolate, marshmallow creme, unsweetened chocolate, and vanilla extract in a medium heat proof bowl.
In a heavy large saucepan (4 to 6 quarts), mix water and espresso powder until the espresso powder dissolves. Add the sugar, sweetened condensed milk, whipping cream, and unsalted butter, and stir over medium heat until sugar dissolves, brushing down sides of pan occasionally with wet pastry brush.
Attach a clip-on candy thermometer to the side of pan. Increase heat to high and bring the mixture to a boil. Reduce heat to medium, and stir constantly but slowly with wooden spoon until candy thermometer registers 234°F, about 12 minutes.
Immediately pour the hot candy mixture over the ingredients in bowl (do not scrape pan).
Stir mixture vigorously with wooden spoon until all chocolate melts and fudge thickens slightly, about 3 minutes.
Transfer the fudge mixture to prepared baking dish. Smooth the top of fudge with an offset spatula. Refrigerate fudge, uncovered, until firm enough to cut, about 2 hours.
Using aluminum foil as aid, lift fudge from pan. Cut into 32 pieces.
Fudge can be prepared a week ahead. Store in airtight container in refrigerator. Bring fudge to room temperature before serving.
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