Into a large mixing bowl, add the eggs, vegetable oil, kosher salt, vanilla extract, granulated sugar, and espresso powder, and sift the cocoa powder. Mix with a whisk until well combined.
Add the flour, cornstarch, and baking powder, and mix until the mixture is just combined and there are no lumps of flour visible. Do not overmix!
Cover the bowl with plastic wrap and let the mixture chill in the fridge for at least 2 hours. (I like letting it chill in the freezer for an hour for a quicker process) The mixture is very sticky so this step is important so you can roll the balls.
Use a cookie scoop to scoop out the cookies or scoop the cookies with a regular tablespoon, 1 ½ tablespoons at a time.
Roll into a ball using your clean hands, and then roll each dough ball in granulated sugar and then powdered sugar.
Place the cookie dough balls onto a parchment paper-lined baking sheet, and bake in a 180c (350f) preheated oven for 10-12 minutes, depending on the size of your cookies.
Do not overbake these cookies to ensure that they are fudgy on the inside.
Let the cookies cool for 5 minutes, and then transfer them to a wire rack to cool completely.