Preheat oven to 350°F. Line baking sheet with parchment paper.
In a microwavable bowl, melt butter and chocolate chips until melted and smooth. Stir in 30 second intervals. Set aside.
In the bowl of a stand mixer with the whisk attachment, add the granulated sugar, light brown sugar, vanilla extract, instant espresso and eggs. Whisk on medium-high speed until combined and pale in color, about 2-3 minutes. Add cooled chocolate and mix until combined.
In medium bowl, sift together the flour, cocoa powder, baking powder and salt. Using a spatula, add the dry ingredients to the wet ingredients and mix until just combined.
Once ready to make the cookies, preheat oven to 350°F. Line a baking sheet with parchment paper. Scoop the dough in tablespoon-sized balls and place each ball 2 inches apart on baking sheet.
Bake for 10-12 minutes or until the tops begin to slightly crack. Allow the cookies to cool for 5 minutes before transferring to a cooling rack. Sprinkle with some flaky sea salt if desired.
Notes
Ensure that ingredients like eggs are at room temperature. Room temperature ingredients incorporate more easily, resulting in a smoother batter.