Preheat your oven to 350ºF.

In a medium bowl, combine the flour, cornstarch, baking powder, baking soda and salt. Whisk thoroughly until combined. Set aside.

In a separate large bowl, combine the butter and the sugars. Use either a handheld mixer or a stand mixer, and cream the butter and sugars together until light and fluffy and sufficiently mixed (1-3 minutes).

Add the eggs to the butter and sugar mixture, one at a time, beating each time until fully incorporated.

Using a sift or a fine mesh sieve, sift the cocoa powder into the large bowl with the wet ingredients. Sifting keeps out the pesky clumps that cocoa powder is prone to having. Mix until all the cocoa powder is incorporated.

Add the flour mixture to the wet ingredients. Mix until the flour is just about totally combined give or take a few streaks of powder. At this point, add in your chocolate chips and stir until the chocolate chips are evenly distributed across the batter.

Using your hands or a cookie scoop, divide the dough into balls, roughly 1-2 tablespoons in size.

When all cookies have been given their thumbprints and half of marshmallows, line a baking sheet with parchment paper and bake the cookies for 10-12 minutes until the middle no longer looks wet/raw. Note: a good spacing is 12 cookies per pan.