Preheat your oven to 350 degrees. If possible, line 2 baking sheets with parchment paper so you can bake the cookies in one batch, if not separate batches is fine.
In a microwave safe bowl combine the chocolate chips, butter & espresso powder. Heat in the microwave for 30 seconds and then remove the bowl and give the ingredients a quick stir. Return the bowl to the microwave to heat for another 30 seconds, repeat process until the chocolate chips and butter are completely melted and smooth, be careful not to over do it so you don't burn the chocolate. Set the melted chocolate in the refrigerator to cool.
In a large mixing bowl combine the sugar and eggs. Using a handheld electric mixer or a stand mixer with a whisk attachment, beat the sugar and eggs together for 5 minutes on med-high speed. Do not skimp on time. After 5 minutes grab the chocolate from the refrigerator and slowly pour it into the sugar/egg mixture and add in the vanilla extract. Mix together on low speed just until combined. Switch to a paddle attachment (if using stand mixer) and slowly add the flour, baking powder & salt to the wet ingredients, mixing slowly and just until combined. The cookie dough will be thick and sticky, not at all firm.
Use a cookie dough scoop to scoop out cookie dough and place drops of dough onto the baking sheets, leaving about 2 inches of space in between each cookie. Bake for 10-12 minutes, but closer to 10. Sprinkle tops of cookies with flaked sea salt. Cookies will be slightly inflated when they come out of the oven but they will deflated as they cool. Allow cookies to cool for about 10-15 minutes before moving them to a cooling rack.