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, plus the vanilla and beat to combine. 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 rounded Tablespoonfuls of dough onto the baking sheets about 2" apart. I used a 2-ounce cookie scoop, but if you don't have one, simply use a 1/4 cup measuring cup and stack the cookie dough higher rather than wider so when it spreads, it stays puffy in the middle.

Bake for approx. 9-12 minutes, rotating pans halfway through baking time to ensure even cooking. Cookies may appear slightly soft in the center - they will continue to cook as they cool. Do not over-bake! Cool completely on the cookie sheets before serving.