Combine dry ingredients (flour blend, cocoa powder, salt, and baking powder) in a small bowl and set aside.
In a large bowl, beat butter until smooth. Add powdered sugar and cream until fluffy. Add egg white, vanilla and peppermint and beat until combined. Mixture will look a little curdled.
Dump dry ingredients into bowl and beat on low speed until well combined.
Wrap dough in plastic wrap and chill for at least 2 hours.
Preheat oven to 350° F. Line a baking sheet with parchment paper and set aside.
On a floured surface, roll out dough to about ⅛ of an inch thick. If you feel more comfortable, you can roll out dough between two sheets of parchment.
Cut with 1¾ to 2-inch round cookie cutter. Place rounds on baking sheet spaced one inch apart. They won't spread much at all.
Bake for 8-10 minutes, or until cookies are slightly firm to the touch and fragrant. You won't be able to tell by color.
Place on wire rack and cool completely.
Melt chocolate in the microwave at 30 second intervals, stirring after every interval. Add chopped cocoa butter (or oil, if using) and stir to combine well (or until cocoa butter is fully melted).
Dip cookies in chocolate using dipping fork or dinner fork and place back on parchment-lined baking sheet. Allow to set at room temperature (or for faster setting, refrigerate until set). Cookies will last several days at room temperature, or can be frozen for up to 3 months.