Preheat the oven to 350°F. Line baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cocoa, baking powder, and salt. Set aside.
In a large bowl, use an electric mixer to combine the butter and sugar until light and fluffy. Beat in the egg and vanilla. Gradually mix in the flour mixture and mix just until combined.
Scoop out the dough by the teaspoon and form balls; place on prepared baking sheets about 2 inches apart (they won't spread much). Dip the bottom of a glass in water, and flatten the balls into 1½ inch rounds (about ¼ inch thick). Bake until slightly firm to the touch, 8 to 10 minutes, rotating sheets halfway through. Immediately transfer cookies to a wire rack to cool completely. Keep parchment on baking sheets.
DIPPING
Place the chocolate, peppermint extract and salt in a glass microwave proof pyrex bowl or measuring cup. Heat in the microwave in 30 second intervals, stirring every 30 seconds, until chocolate is smooth and hot. Remove from the microwave and set next to parchment lined baking sheets. (If you do not have a microwave, you can melt the chocolate in a small pan over very low heat, stirring constantly until melted.)
Set each cookie across the tines of a fork and dunk in the chocolate. Tap the underside of the fork on the side of the bowl to allow excess chocolate to drip off and create a smooth top. Gently place cookies on baking sheet, and decorate with sprinkles (if desired).
Refrigerate until the chocolate has hardened, about 30 minutes, and keep chilled until ready to serve.