Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

Preheat oven to 375 degrees. Line additional baking sheets with parchment paper or nonstick baking mats.

Step 6

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.