Preheat oven to 350 degrees. Line baking sheets with parchment paper, set aside.
In a medium bowl, whisk together the flour, cornstarch, baking power, and salt.
In a small bowl, whisk together the eggs, brown sugar, vanilla extract, and espresso powder.
Using a double boiler or a large bowl set over a pot of barely simmering water, stir the chopped chocolate and butter until melted and smooth. Remove from the heat and allow to cool until it's warm, but not hot, about 5-10 minutes.
Add the egg mixture to the chocolate mixture, stir to combine. Gradually mix in the dry ingredients, then fold in the chocolate chips.
Using a standard size cookie scoop, place dough onto the cookie sheets, leaving 2 inches between each ball of dough (they do spread a lot).
Place in the oven and bake for 10-12 minutes. The edges should be set but the centers should be slightly gooey. Remove from the oven and allow to cool of the baking sheets for 3-4 minutes before transferring them to a wire rack to cool completely.
Notes
*Just make sure your baking powder is relatively new **I like to use the Ghirardelli baking bars, they're 4 oz. each. -You can also add chopped toasted nuts or a swirl of peanut butter into the batter!