Place cornflakes in a bowl and crush with your hands until they are 1/4 of their original size.
Add milk powder, sugar, and salt. Toss to mix. Add melted butter and toss to coat.
Spread cornflakes on a silpat or parchment lined sheet pan (I used 2 half-sheet pans).
Bake for 20mins or until they are toasted and smell buttery.
Cool completely before adding to recipe.
Measure out 3 cups of cornflake crunch and set aside.
Cookies
Place butter and sugars into the bowl of a stand mixer fitted with the paddle attachment. Cream on med-high for 2-3mins.
Scrape bowl and add the egg and vanilla. Beat on med-high for 7-8mins (DO NOT skimp on the mixing time, do the full 7-8 mins).
In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
Reduce mixer to low and add in dry ingredients. Mix just until dough comes together, 30-45 secs. DO NOT overmix.
Add cornflake crunch and chocolate chips and mix until just incorporated.
Add marshmallows and mix until just incorporated.
Using a large ice cream scoop (2 3/4oz, 1/3cup), portion the dough onto a parchment lined sheet pan. You can place them quite close together for this phase. (I used a med cookie scoop because that's what I had).
Press down on the tops of each ball to flatten a bit, so they look more like hockey pucks.
Wrap sheet pan in plastic wrap and refrigerate at least 1hr (or freeze for 30mins).
Preheat oven to 350°F.
Place cookies on a parchment lined cookie sheet at least 4" apart. I did 5 cookies per half sheet pan and baked one pan at a time. Bake for 12-15mins.
Cookies will spread a lot and be very soft right out of the oven. Cool on sheet pan for 10mins then gently transfer each cookie to a wire rack to cool completely.
Notes
Do not skimp on the mixing time. Do not bake cookies from room temperature, they will not turn out. Refrigerate at least 1 hour or up to 1 week. Bake cookies on parchment rather than silpat. Silpat can cause the cookies to spread too much. The original recipe calls for baking the cookies at 375°F for 18mins. I (and many other bloggers) baked at 350°F for 12mins and they turned out perfect.https://milkbarstore.com/main/press/recipes-and-how-tos/#cornflake