Melt the milk chocolate in a heatproof bowl set over a pan of barely simmering water, stirring until smooth, about 4 minutes.

Spoon about a teaspoon of melted chocolate into the bottom of each cavity of a 12 cup silicone heart mold, tilting to coat the bottom and slightly up the sides.

Refrigerate the mold for 10 minutes, until the chocolate layer is fully set and cool to the touch.

In a small bowl, stir the peanut butter, powdered sugar and salt together until smooth and thick enough to hold a shape.

Spoon about a teaspoon of the peanut butter mixture into each chocolate-lined cavity, pressing it flat and leaving room at the top for a final layer.

Gently rewarm the remaining chocolate if it has thickened, then spoon it over the peanut butter layer in each cavity, covering it completely.

Sprinkle a few flakes of sea salt over the tops while the chocolate is still wet.

Refrigerate for 30 minutes, until fully firm, then press gently on the back of the mold to release each cup.