Set up your double boiler by filling a pot with about 2 inches of water and bringing it to a simmer. Place a heatproof bowl on top, ensuring the water doesn’t touch the bottom of the bowl.
Chop or break the chocolate into smaller pieces to help it melt evenly, then place it in the bowl. Stir constantly with a spatula as it melts, until glossy and smooth, about 3-5 minutes.
Using a spoon or a small brush, pour a spoonful of melted chocolate into each mold or liner, tilting gently to cover the bottom evenly. Place the molds in the fridge for about 10 minutes until the layer sets and becomes shiny.
In a small bowl, mix the peanut butter with the melted butter or coconut oil, a pinch of sea salt, and optional sweetener if desired. Stir until the mixture is smooth and creamy.
Remove the molds from the fridge and spoon a small amount of peanut butter mixture into each chocolate shell, pressing down gently to fill most of the cavity but leaving some space at the top.
Top each filled mold with the remaining melted chocolate, pouring or spooning over the peanut butter to seal it. Tap the molds lightly on the counter to remove air bubbles and ensure an even layer.
Place the molds back into the fridge for at least 30 minutes until the chocolate is fully firm and shiny.
Once set, carefully pop the peanut butter cups out of the molds or peel away liners. Optionally, sprinkle a tiny pinch of sea salt or chopped nuts on top for added flavor and crunch.