Line a 9x5 inch loaf pan with parchment paper, leaving overhang on the long sides. Place the chopped chocolate in a large heatproof bowl.----------------
In a small saucepan, combine the butter, heavy cream, powdered allulose, espresso powder and salt. Heat over medium, stirring, until the butter melts and the mixture just begins to simmer around the edges.----------------
Pour the hot cream mixture over the chocolate and let it sit undisturbed for 2 minutes to melt.----------------
Stir slowly from the center outward until the chocolate is completely melted, thick and glossy. Stir in the vanilla. If any small lumps remain, microwave for 15 seconds and stir again.----------------
Pour into the prepared pan, smooth the top, and refrigerate for at least 3 hours until firm.----------------
Lift the fudge out using the parchment, dust the top with cocoa powder and sprinkle with flaky sea salt. Cut into 16 squares and store in the fridge for up to 2 weeks.