Make Ahead & Freezing Instructions: You can make the cookie dough and chill it in the refrigerator for up to 3 days or freeze for up to 3 months. Allow to come to room temperature and continue with step 4. Baked and cooled cookie bars freeze well for up to 3 months. Thaw bars overnight in the refrigerator and bring to room temperature before serving.
Special Tools (affiliate links): 9×13-inch Glass Baking Pan or Metal Baking Pan | Parchment Paper | Glass Mixing Bowls | Whisk | Silicone Spatula | Cooling Rack
Cornstarch: If you don’t have cornstarch, you can leave it out. The cookie bars will still be soft.
Butter: Avoid letting the melted butter cool for too long, otherwise your dough will be crumbly instead of soft (and your cookie bars can end up cakey). You want it still warm, but not hot enough to begin cooking the eggs.
M&Ms: You can use regular-size, mini, or a mix of both. I like to crush some of them, too, to sprinkle on the top before baking. Totally optional!
Do I have to add chocolate chips? You can skip the chocolate chips. If you skip them, increase M&Ms to 1 and 1/2 cups (about 300g). You could also replace the chocolate chips with white chocolate chips, butterscotch chips, or peanut butter chips.
9-Inch Square Pan: To make a smaller 9-inch pan of cookie bars, use this similar scaled-down recipe for chocolate chip cookie bars. For the add-ins, use 1 cup (200g) of M&Ms and 1/2 cup (90g) chocolate chips.
