Begin by melting the chocolate in a microwave-safe bowl, heating in 30-second bursts and stirring in between until silky, glossy, and smooth.
Pour about a teaspoon of melted chocolate into each mold or paper liner, gently tilting to spread the chocolate evenly and form a thin base.
Chill the molds in the fridge or freezer for about 10 minutes until the chocolate base is set and slightly firm.
While the chocolate chills, mix the peanut butter with a pinch of salt and powdered sugar until smooth and creamy, adjusting for sweetness if desired.
Once the chocolate is firm, dollop about a teaspoon of the peanut butter mixture into the center of each chocolate base, gently pressing it down.
Cover the peanut butter layer with another teaspoon of melted chocolate, ensuring the filling is sealed completely and the surface is smooth.
Gently tap the molds on the counter to remove air bubbles and even out the surface, then sprinkle with flaky sea salt flakes if using.
Refrigerate the cups for at least 30 minutes until fully set and firm.
Carefully peel away the liners or gently pop the silicone molds to release the peanut butter cups, revealing their glossy, smooth surface and inviting layers.
Serve at room temperature for best flavor, or store in an airtight container in the fridge for up to two weeks, enjoying the satisfying crunch and rich chocolate taste.