Heat half of the coconut milk and all of the gelatin in a small, heavy-bottomed saucepan over low-medium heat, whisking until the gelatin is dissolved, and bring to a gentle simmer. Add the butter and stir.
Remove from the heat, and add the remaining coconut milk, Erythritol, lemon zest, and vanilla extract. Stir until well combined.
Pour the mixture into four ramekins and place in the refrigerator for 2-4 hours to set.
Heat the berries in a saucepan until warm, and optionally add extra sugar-free sweetener. Mix until you get a smooth berry sauce.
Place the ramekins upside down to remove the panna cotta. Serve cold, topped with a sprinkle of lemon zest, fresh berries, and berry sauce.