Chop the dark chocolate into small, uniform pieces to ensure even melting. Place the chopped chocolate in a heatproof bowl.
Set up a double boiler by placing the bowl over a pot of simmering water. Stir the chocolate constantly with a silicone spatula until it melts smoothly and is fully glossy, reaching about 60°C (140°F).
Line a mini muffin tin with paper liners, then spoon about a teaspoon of melted chocolate into each liner. Use the back of the spoon to spread and evenly coat the bottom, creating a thin layer.
Place the muffin tin in the fridge for 10 minutes, allowing the chocolate to set and firm up slightly.
While the chocolate chills, mix the peanut butter with a pinch of sea salt and vanilla extract in a small bowl until smooth and slightly creamy.
Remove the muffin tin from the fridge and spoon about a teaspoon of peanut butter mixture on top of the set chocolate layer in each cup. Gently press down to level the surface.
Cover the peanut butter layer with another teaspoon of melted chocolate, ensuring the peanut butter is sealed inside. Use a spatula to smooth the top if needed.
Chill the assembled cups in the fridge for at least 30 minutes until the chocolate is glossy and fully firm.
Once chilled, gently remove the peanut butter cups from the liners and place on a plate. If desired, sprinkle chopped nuts or sea salt on top for added texture and flavor.
Let the peanut butter cups sit at room temperature for a few minutes before serving to soften slightly for the perfect bite.