In a small bowl, stir together the flour, salt, baking soda, and cornstarch. Set aside.
Place butter and both sugars in a separate bowl and beat with an electric mixer until light and creamy, about 2 minutes.
Beat in the eggs and vanilla until just incorporated.
Stir in the flour mixture until just combined, being careful not to overmix.
Stir in the marshmallows, rice krispies, and chocolate chips, as desired.
Use a medium cookie scoop to portion the cookie dough into 1 Tablespoon portions onto a prepared baking sheet.
Bake cookies for 8-10 minutes, until lightly golden brown, avoiding burning the marshmallows. Cool on a cooling rack.
Notes
Watch the Oven: Oven temperatures can vary, so start checking your cookies around the 8-minute mark. They should be lightly golden around the edges but still soft in the center.
Room Temperature Ingredients: For the best texture, ensure that your butter and eggs are at room temperature before you start mixing.
Don’t Overmix: Once you add the flour, mix just until it's incorporated. Overmixing can lead to tough cookies.
Even Distribution: To ensure every perfect cookie gets an even mix of marshmallows, Rice Krispies, and chocolate chips, give your dough a final stir by hand before scooping.
Marshmallow Watch: To keep marshmallows from sticking to the pan or burning, try to tuck them into the middle of the dough balls when scooping.
Cooling Time: Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack. This helps them set and makes them easier to move without breaking.
Cookie Scoop: Using a standardized cookie scoop ensures even cooking and uniform cookie size for a professional bakery look.
Parchment Paper or Silicone Mats: Line your cookie sheets with parchment paper or silicone baking mats for easy release and clean-up.