In a large bowl of a stand mixer, cream together the butter, brown sugar and sugar with a paddle attachment until creamy, about 1 minute.
Add in the egg and egg yolk and beat. Add in the espresso powder, soda, cornstarch, salt and flour and beat until a soft dough forms. Fold in the chocolate chips.
Chill the mixture covered in the fridge for at least 2 hours, up to 3 days long. Chilling is mandatory and prevents spreading the cookie dough. Preheat oven to 350 degrees F.
Line 2 baking sheets with silicone liners or parchment paper. Drop cubes of dough onto the baking sheets. I only bake 5 cookies at once to avoid that they stick together. Cut your chilled dough in cubes, they should be around 1/4 sup each and stack the cookie dough higher rather than wider so when it spreads, it stays puffy in the middle. Bake for approx. 10 minutes, rotating pans halfway through baking time to ensure even cooking. Cookies may appear slightly soft in the center; they will continue to cool as they bake. It’s very important to not over-bake! Cool completely on the cookie sheets before serving.