Preheat the oven to 350 degrees F. Line two baking sheets with silpat mats or parchment paper.
In a large bowl, use an electric mixer to combine the butter, brown sugar and Stevia. Mix until well combined. Beat in the eggs and vanilla.
In a separate bowl, whisk together the flour, cocoa powder, cornstarch and baking soda. Add it to the wet ingredients and beat until the dough is smooth.
Scoop out a tablespoon of dough, divide it into two pieces and flatten each piece with your hands. Place a chocolate peppermint between the two pieces of dough and use your hands to wrap the dough completely around the mint.
Roll the sides of the stuffed cookie and the top of the cookie in sprinkles. Place the cookie on the prepared baking sheet, sprinkle-side-up. These cookies do not spread out much, so you can place them about 1½ inches apart. Repeat until you've used up all of the dough and mints.
Bake for 10 to 12 minutes, just until cookies are set and beginning to crack on the tops. Let cool for several minutes on the baking sheet before transferring to a wire rack to cool completely.
Notes
These cookies may be stored at room temperature in a covered container for at least a few days. They freeze wonderfully, so I suggest keeping them in the freezer until ready for munching or sharing.