Preheat the oven to 350 F/180 C. Line two large baking sheets with parchment paper and set them aside.
In a microwave-safe bowl (or in a pot over low heat on the stove), melt the chocolate in 20-second increments, stirring in between until fully melted. Set aside to cool for 5 minutes while you prep the rest of the ingredients.
In a medium bowl whisk together the flour, cocoa powder, espresso powder, baking powder, and salt. Set aside.
In another glass heat-proof bowl, combine the butter and sugars. Microwave for 1-2 minutes, just long enough to melt the butter. Once melted, combine fully with a whisk.
Whisk the eggs, egg yolk, and vanilla into the butter and sugar mixture until combined.
Pour the melted chocolate bar into the bowl. Fold in with a spatula until combined. Then fold in the dry ingredients until just combined.
Using a large (3 tbsp) cookie scoop - place 6-7 cookies spaced 2 inches apart on the prepared baking sheets. Bake one tray at a time (leaving the other on the counter) for 9-10 minutes. For a standard (2 tbsp) cookie scoop - place 9-10 cookies on each tray. Sprinkle the tops with a pinch of flaky sea salt. Bake for 8-9 minutes. Place the tray on a wire rack to cool, then bake the other tray.Make sure to scoop the entire bowl of batter right away, if it sits in the bowl the cookies will not have shiny crinkle tops. Store leftover cookies in an airtight container at room temperature for 2-3 days.