For the cornflake crunch: Heat the oven to 135°C.

Pour the cornflakes in a medium bowl and crush them with your hands to one-quarter of their original size. Add the milk powder, sugar, and salt and toss to mix. Add the butter and toss to coat. As you toss, the butter will act as glue, binding the dry ingredients to the cereal and creating small clusters.

Spread the clusters on a parchment- or Silpat-lined sheet pan and bake for 20 minutes, at which point they should look toasted, smell buttery, and crunch gently when cooled slightly and chewed.

Cool the cornflake crunch completely before storing or using in a recipe. Stored in an airtight container at room temperature, the crunch will keep fresh for 1 week; in the fridge or freezer, it will keep for 1 month.

For the cookies: Combine the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add the egg and vanilla, and beat for 7 to 8 minutes.

Reduce the mixer speed to low and add the flour, baking powder, baking soda, and salt. Mix just until the dough comes together, no longer than 1 minute.

Still on low speed, paddle in the cornflake crunch and mini chocolate chips just until they’re incorporated, no more than 30 to 45 seconds.

Using a ⅓-cup ice cream scoop, portion out the dough onto a parchment-lined sheet pan. Pat the tops of the cookie dough domes flat. Press on marshmallows to the middle of the cookie. Wrap the sheet pan tightly in plastic wrap and refrigerate for at least 1 hour, or up to 1 week. Do not bake your cookies from room temperature—they will not hold their shape.

Heat the oven to 180°C (youtube) instead of 190°C.

Arrange the chilled dough a minimum of 4 inches apart on parchment- or Silpat-lined sheet pans. Bake for 10 minutes (18 in recipe). The cookies will puff, crackle, and spread. they should be browned on the edges and just beginning to brown toward the center. Leave them in the oven for an additional minute or so if they aren’t and they still seem pale and doughy on the surface.

Cool the cookies completely on the sheet pans before transferring to a plate or to an airtight container for storage. At room temperature, the cookies will keep fresh for 5 days; in the freezer, they will keep for 1 month.”