Mix the filling ingredients together by hand or in a processor until smooth. The mixture should be soft but not runny. If needed, mix in additional MCT oil until the mixtures reaches the desired consistency. Mix in sweetener as desired. Drop spoonfuls onto a small lined baking tray to create 6 mounds or discs. Freeze the tray while you work with the chocolate.
Melt the chocolate for the first time: either place the chocolate and MCT oil in a glass measuring cup and heat in the microwave in 30 second increments, or place in a double boiler or Bain Marie over medium heat until just melted. Either way, stir often and stop as soon as the chocolate is melted, remove from heat, and set aside.
Using half your melted chocolate, pour equal amounts into 6 muffin cups. Go halfway up the sides with chocolate, either by swirling up the sides or using a silicone brush or other utensil. Chill the chocolate cups until firm, about 1 hour. The chocolate will look less shiny and will not stick to a utensil or finger if prodded
Once the chocolate firms up, pull them and your frozen fillings back out. Place one filling in the center of each chocolate cup. Pour the remaining chocolate equally over the nut butter in the 6 filled cups, using a utensil to get the chocolate in the cracks and seal the nut butter centers in. If necessary, gently melt the remaining chocolate again first, being careful not to scorch it.
Place your nut butter cups back in the freezer briefly until the chocolate is firm, then enjoy them, or place them into a sealed container and refrigerate for later.