Line a muffin tin with 12 paper liners.
Melt the chocolate by placing in a heatproof bowl set over a pan of gently simmering water, stirring occasionally until melted. Alternatively place the heatproof bowl in the microwave at 30 second intervals, stirring between each one, until melted.
14 ounces dark chocolate (50-60% cocoa solids)
Spoon 2 teaspoons of melted chocolate into the bottom of each muffin liner. Using the back of a small spoon or spatula, carefully spread the chocolate up the sides of the liner, about 1 cm/½ inch or so. Place the muffin tray in the freezer to allow the chocolate to set.
Meanwhile, place the cashew butter into a small bowl. Add the maple syrup, vanilla and salt and stir with a spoon until a dough forms. Divide the dough into 12 balls (about 2 teaspoons each) and then flatten into patties just slightly smaller than the base of your muffin liners.
½ cup cashew butter, 2 tablespoons pure maple syrup, 1 teaspoon vanilla extract, flaked sea salt
Remove the muffin tin from the freezer and place the cashew butter patties into each of the muffin liners. Spoon the remaining chocolate on top of each one, starting with another 2 teaspoons and adding more if you have leftovers. Your chocolate should still be melted by this point but if it's set a little, just set it back over the pan of simmering water for a couple of minutes.
Gently level the tops of the cashew butter cups using the back of a spoon. If you'd like to do a cashew butter swirl on top, in a small bowl, mix 1 tablespoon of cashew butter with ½ teaspoon of a neutral flavoured oil. You should be able to drizzle it now but if not, add a touch more oil. Dot the tops of the cups with the runny cashew butter and gently swirl using a toothpick or cocktail stick. Sprinkle with sea salt and return to the freezer to set completely.
When set, remove the cashew butter cups from their paper liners and store in an airtight container or freezer bag in the fridge or freezer.