In a medium saucepan, combine the heavy cream, sugar, and corn syrup for the chocolate layer. Heat over medium, gently swirling the pan to dissolve the sugar evenly into the cream, preventing crystallization. Heat until the mixture reaches 240°F (hard-ball stage), using a candy thermometer. If it bubbles too much, briefly remove it from the heat, then return it to the burner.

Remove the saucepan from heat and stir in the chopped unsweetened chocolate until fully melted and smooth. Let the mixture cool undisturbed to 110°F to avoid graininess. Repeat these steps for the gingerbread layer, substituting molasses for corn syrup, and using milk chocolate instead of unsweetened chocolate.

Once both fudge mixtures have cooled to 110°F, transfer each to separate mixing bowls. For the chocolate fudge, add room-temperature butter, vanilla extract, and salt, then mix on low speed until the fudge thickens and turns matte (about 5-8 minutes). For the gingerbread fudge, mix in the butter, vanilla, spices (cinnamon, ginger, nutmeg, cloves), and salt. Aerate the gingerbread fudge the same way, until it thickens and loses its shine.

Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal. Pour half of the aerated chocolate fudge into the pan, spreading it evenly. Drop spoonfuls of both the remaining chocolate and gingerbread fudges over the base layer, then swirl them together using a toothpick or skewer to create a marbled effect.

Let the fudge sit at room temperature for 2-3 hours until fully set. Avoid refrigerating, as this can make the fudge too firm or grainy.

Once set, lift the fudge out of the pan using the parchment overhang. Cut into squares with a sharp knife, warming the knife slightly for clean edges. Store any leftovers in an airtight container at room temperature for up to two weeks.