Preheat oven to 350° F. Line a baking sheet with parchment paper.
In a small saucepan over medium-low heat, melt the chocolate and butter together until smooth. Set aside to slightly cool.
Using a whisk or an electric hand mixer on medium, beat the egg, sugar and salt in a medium bowl until thick and light in colour. Add in the melted chocolate and stir until combined.
Sift in the corn starch, cocoa powder, espresso powder and baking powder. Stir to combine.
Add in the flavourings and stir one last time. Cover and refrigerate until stiff, about 30 minutes.
Scoop about 1 1/2 tbsp of batter per cookie onto prepared baking sheet. Bake for 10 minutes or until set. Let cool completely on baking sheet. The cookies will still be soft and gooey when you first take them out of the oven, but will set into a fudge-like texture as they cool. Enjoy!
Store in a sealed container at room temperature for up to 3 days.