Line a small 8x4 inch loaf pan with parchment paper, leaving an overhang on the long sides for easy removal.
In a medium bowl, stir together the peanut butter and melted coconut oil until completely smooth and well combined.
Add the powdered erythritol, vanilla extract, and salt. Mix until the sweetener is fully dissolved and the mixture is uniform.
Pour the fudge mixture into the prepared pan and spread it into an even layer with a spatula. Sprinkle the top with flaky sea salt.
Freeze for at least 2 hours, or until the fudge is completely firm and solid.
Lift the fudge out using the parchment overhang and cut into 16 small squares. Store in the freezer or refrigerator, as the fudge softens quickly at room temperature.