Preheat oven to 325°F with convection // 350°F without fan // 180°C // Gas Mark 4.
Combine flour, baking powder and salt in a medium bowl. Whisk to combine, then set aside.
In the bowl of a stand mixer, or a large mixing bowl, beat butter, brown sugar and granulated sugar together. Then add eggs, one at a time. Beat until thoroughly combined.
Scrape down bowl, then add peppermint extract and mix to combine.
Add dry ingredients and peppermint candy, if using. Mix to combine well, then remove dough from the bowl and divide in half. (If weighing, each ball of dough should weigh just over 15 ounces)
Form 2 equal loaves of dough roughly 3" wide by 8 inches long and 3/4" thick on a parchment lined baking sheet. *If your dough is too warm, chill for 20 - 30 minutes before baking.
Bake for 22-25 minutes. Remove from oven and let cool for 15 minutes. Slice logs into 1/2" slices and lay them on their sides.
Return the biscotti to the oven and bake for an additional 15 - 20 minutes. Cool completely on cooling rack before drizzling with chocolate (see note!).