1. Line an 8-inch square baking dish with aluminum foil, smoothing surfaces as much as possible.

2. Place the heavy cream and the cardamom, if using, in a small saucepan, and bring to a boil. Remove from heat, and let steep for 15 minutes.

3. Place the bittersweet chocolate, unsweetened chocolate, marshmallow cream, and vanilla extract in a large heat-proof metal bowl.

4. Strain the heavy cream through a fine sieve into a 3-quart saucepan, discarding cardamom. Add the sugar, sweetened condensed milk, water, and butter. Stir the mixture over medium-low heat until the sugar dissolves. Wash down the sides of the pan with a pastry brush dipped in water to prevent sugar crystals from forming. Raise the heat to high, and bring the mixture to a boil. Attach a candy thermometer to the pan, and reduce the heat to medium-high. Stir mixture slowly with a wooden spoon until a candy thermometer registers 232 degrees.

5. Remove pan from heat, and pour boiling mixture over chocolate mixture in bowl; do not scrape pan. Stir vigorously with a wooden spoon until the mixture is well combined and glossy. Pour the mixture into prepared pan, and smooth the top with a small offset spatula. Allow the fudge to set in refrigerator until firm, about 2 hours.

6. Cut fudge into 1 1/4-inch rounds using a cookie cutter. Alternatively, you may cut into squares with a sharp knife. If desired, cut a small heart stencil out of parchment or waxed paper. Place the stencil over a piece of fudge, and lightly sift cocoa powder over it. Remove the stencil, and repeat with the remaining fudge.