Instructions

1. Cookie Instructions:
2. Cream together butter and both sugars in the bowl of an electric mixer fitted with the paddle attachment on medium-high, 2 to 3 minutes. Scrape down sides of bowl and add egg and vanilla; beat for 7 to 8 minutes.
3. Reduce speed to low and add flour, salt, baking powder, and baking soda. Mix just until dough comes together, no longer than 1 minute. Scrape down sides of bowl with a spatula.
4. With the mixer on low, add cornflake crunch and chocolate chips; mix until just combined, 30 to 45 seconds. Add mini marshmallows and mix until just incorporated.
5. Line a baking sheet with parchment paper. Using a 2 3/4-ounce ice-cream scoop or 1/3-cup measuring cup, portion dough out onto prepared baking sheet. Pat tops of cookie domes flat. Wrap baking sheet tightly with plastic wrap and refrigerate for at least 1 hour and up to 1 week. Do not bake cookies from room temperature or they will not hold their shape.
6. Preheat oven to 375 degrees. Line additional baking sheets with parchment paper.
7. Arrange chilled dough at least 4 inches apart on prepared baking sheets. Transfer to oven and bake until puffed, cracked, spread, and browned on the edges, about 18 minutes. Let cool completely on baking sheets.
8. Cornflake Crunch instructions:
9. Preheat oven to 275 degrees. Line a baking sheet with parchment paper; set aside.
10. Place cornflakes in a medium bowl. Using your hands, crush to one-quarter of their original size. Add milk powder, sugar, and salt; toss to combine. Add butter and toss to form small clusters.
11. Spread cornflake mixture in an even layer on prepared baking sheet. Transfer to oven and bake until clusters are toasted and smell buttery, about 20 minutes. Remove from oven and let cool completely.