Line two mini muffin tins with paper liners and lightly spray with cooking spray. Set aside.
Melt 2/3 of the chocolate in a double boiler. Once the temperature reaches 115 F, remove from heat and stir in the remaining 1/3 chocolate. Stir until all chocolate is melted.
Spoon enough chocolate to cover the bottoms of the muffin liners with a thin layer of chocolate. Freeze or refrigerate until firm.
Meanwhile, purée pistachios in a food processor until very fine and mushy. Add enough almond or pistachio oil to form a thick paste. (Can also use coconut oil.)
Using a round half-teaspoon measuring spoon or small melon baller, form pistachio butter into small balls and slightly flatten into a disc that will almost fill the width of the muffin tins, allowing a little room around the edge to pour the final chocolate shell. Chill until firm.
Place pistachio discs on top of the chocolate bottom in the muffin tins. Spoon remaining melted chocolate (you may need to re-melt the chocolate) over the pistachio filling, allowing the chocolate to completely cover the top and sides of the pistachio butter. Freeze or chill for about 10 minutes and then bring them out of the refrigerator.
Lightly dust tops of finished butter cups with cocoa powder and sprinkle the sea salt over the top. I actually just use my Himalayan pink salt grinder for this part, grinding a little salt over the top of each pistachio cup. Place back in the refrigerator or freezer until the chocolate is completely firm. Store in the refrigerator or freezer in an airtight container until ready to serve.