Prepare two mini muffin tins with liners and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the powdered sugar, cream, butter, matcha, and peppermint extract. Beat until it is no longer dry, but comes together as a creamy filling.
Roll the filling into a long cylinder, just slightly smaller than the diameter of the bottom of the muffin cups. Wrap it in plastic and chill it while you prepare the chocolate.
Melt the chocolate in a double broiler or in the microwave: fill a small saucepan with a few inches of water and set a heat-proof bowl over the top. Being careful not to get any water in your bowl, add the dark chocolate. Cook, undisturbed until a little more than half is melted, then stir until the rest melts. Cook for a few minutes longer until the chocolate stings a little when touched on your wrist or just below your lower lip.
Remove the bowl from the heat (again being careful not to get any water in it) and let sit, stirring occasionally, until the chocolate feels just a bit cooler than your body temperature when touched to your wrist or just below your lower lip.
Spoon 1 teaspoon chocolate into each cup and use a small spoon to evenly spread it up the sides of the cup leaving about ¼-inch of the cup uncovered. Make sure the bottom of the cup is completely covered. Chill for a few minutes to firm up.
Working quickly, remove the filling and cut the cylinder into 24 rounds. When the chocolate is firm, place one round into each muffin cup so it is not taller than the height of the chocolate shell.
Spoon another teaspoon or so of chocolate over the top of the filling. Spread the chocolate to make sure it goes down around the sides of the filling and smooth out the top. Repeat on all cups.
If desired, wait a few minutes then sprinkle the tops with a dusting of matcha powder and flakey salt while the chocolate is still soft. Let cool completely to harden the chocolate before serving (to speed up the process you can chill them for an hour). Store in an airtight container for up to a week.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.