Line 18 muffin cups with paper liners or use 18 silicone muffin cups.In a medium bowl, mix the tahini (sesame butter) to make sure it's smooth. Sift in the icing sugar. Add a pinch of salt and the cookie crumbs. Mix well to form the sesame butter filling. Set aside in the refrigerator.
Meanwhile, melt the chocolate either in the microwave at power level 5 or over a double boiler. You want to be gentle to avoid burning the chocolate and you want to stir often, especially if you are melting chocolate in the microwave.
When the chocolate is melted and smooth, pour a spoonful into each prepared cup, making sure to tilt and coat the sides with chocolate (about ¼ inch to ½ inch max up the sides.
When all 18 cups are coated with a layer of chocolate, add a dollop (about 2 tsp) of the sesame butter filling
Put the chocolate cups in the freezer and let them chill for a good 10 minutes to set.
Top each cup with a layer of melted chocolate so that the filling is completely covered and sealed inside. Sprinkle with flaky sea salt or sesame seeds, if using. Return the chocolate cups to the freezer to set.
Transfer to a container and store in the fridge.