In a large bowl, whisk together flour, cocoa, baking powder, baking soda, and salt.
Add butter and sugar to the bowl of a standing mixer. Beat on medium-high speed until very light and fluffy, for about 3 minutes.
Scrape downsides of the bowl then beat in eggs, one at a time, then vanilla.
With mixer speed on low, slowly add the flour mixture, and beat until combined.
Cover with plastic wrap and place in the fridge for at least 45 minutes or overnight.
Preheat oven to 175ºC. Place parchment paper on an un-greased baking sheet. Remove the truffle cookie dough from the freezer. Grease your hands with a little canola oil (cooking spray works really well, but water can also be used) then begin to scoop out about 2 teaspoons of dough placing them about 2 inches apart.
Using your oiled (or wet hand) flatten the dough into a small round disk. Repeat with remaining dough. Do this quickly and if you are doing the cookies in batches, refrigerate the dough in between rounds.
Bake 6-8 minutes, until slightly puffed and starting to crackle at edges. Cookies should still be soft and slightly underbaked.
While the cookies are still warm top them with a few squares of unsweetened chocolate and a sprinkle of salt. Transfer to wire racks to cool completely.