Cream the butter and sugar: Place butter and sugar in a large bowl or bowl of a standing mixer with paddle attachment. Whisk together butter and sugar until light and fluffy, about 5-6 minutes (yes, really that long- I've learned from Milk Bar that this is a crucial step that many people overlook and do for far too short).
Add in egg and vanilla extract. Continue to mix at a medium speed for an additional 3-4 minutes, until light and fluffy.
Combine all dry ingredients in a bowl. Add in flour, baking powder, baking soda, and salt into a medium bowl. Stir to combine thoroughly. Add cornflakes on top and then crush with your handles until they are quite crushed and like crumbles. Stir to incorporate into flour mixture.
Add dry ingredients into the bowl with the sugar, butter, and eggs. Mix just enough to incorporate and not see any dry flour bits, but try to minimize how much you mix (overmixing will deflate all the air you just worked to build in with creaming).
Add in chocolate chips and marshmallows and pulse once or twice to evenly distribute.
The next step is to refrigerate the dough for at least an hour or up to three hours.*If you have the space in your fridge, I recommend scooping out the dough into balls, placing onto a baking sheet or plate, and placing the whole sheet/plate in your fridge (see details in step below). If you don't have enough space in your fridge, you can put the whole bowl of dough in the fridge and then scoop the dough into balls after it's been refrigerated.
Using 1/4 cup scooper or measuring cup, scoop dough into twelve 1/4-cup portion balls onto a parchment lined paper (if storing in fridge, you can place them close together, but be sure they are far apart when baking).
Preheat oven to 350. Make sure cookie dough balls are quite far apart from each other- I'll usually put only 6 on a tray. They will spread a lot.
Bake for 16-17 minutes, until lightly golden brown on top and on bottom, but still soft. Store in an airtight container. These are best eaten within about 2-3 days.