Line the square baking pan with parchment paper, leaving overhang on the sides for easy removal.
Combine the heavy cream and dark molasses in a heavy-bottomed saucepan and heat over medium, stirring occasionally, until just simmering and fragrant.
Add the chopped dark chocolate to the simmering mixture, stirring constantly with a rubber spatula until the chocolate is fully melted and the mixture is smooth and glossy.
Stir in the butter and vanilla extract, continuing to stir until the mixture thickens slightly and becomes velvety, about 5 minutes.
Pour the rich fudge mixture into the prepared pan, smoothing the top with the spatula for an even surface.
Let the fudge cool at room temperature for 15-20 minutes, then transfer to the refrigerator to set completely, about 2 hours.
Once set, lift the fudge out of the pan using the parchment overhang and place on a cutting board.
Use a sharp knife to cut the fudge into even squares, revealing its glossy, chewy interior.
Serve immediately or store in an airtight container at room temperature for up to a week.
