Line a muffin pan with the muffin wrappers.
Place the chocolate in a heatproof bowl placed on top of a small saucepan with a few inches of water in it.
Melt on medium high for about 10 minutes.
Pour a centimeter of melted chocolate into the bottom of each muffin paper and leave to cool in the fridge.
Mix the cashew butter, salt, and maple syrup.
Place 1 tablespoon of cashew butter filling in the bottom of each muffin paper on top of the chocolate.
Evenly divide the remaining chocolate by pouring it over the cashew butter in each muffin paper.
Sprinkle fleur de sel on top of each cup if desired then refrigerate until firm, at least one hour.