Step 1

For the toffee: Line a 9-by-9-inch baking pan with foil. Spray the foil with cooking spray.

Step 2

In a medium saucepan, melt the butter over medium heat. Add the brown sugar and bring to a boil, stirring constantly until the mixture forms a golden caramel and the temperature reaches 290°F, 4 to 5 minutes.

Step 3

Carefully pour the mixture into the prepared pan, allowing it to spread. Let the toffee cool and harden for about 20 minutes.

Step 4

Place the toffee sheet in a large zip-top plastic bag and use a rolling pin to break the toffee into small pieces. There should be about 1 1/2 cups of toffee pieces.

Step 5

For the cookie bars: Preheat the oven to 350°F. Line a 13-by-9-inch baking pan with parchment paper, letting the excess extend over the sides of the pan; spray the parchment with cooking spray.

Step 6

In the bowl a stand mixer fitted with a paddle attachment, beat the butter and sugars on medium speed until the mixture has lightened in color and texture, 2 to 3 minutes. Reduce the mixer speed to low. Add the eggs, one at a time, and the vanilla, and beat until smooth, scraping the sides and bottom of the bowl as needed.

Step 7

In another medium bowl, whisk together the flour, cornstarch, baking powder, and salt. Add the flour mixture to the butter mixture in 3 parts, mixing on low speed until fully incorporated after each addition. Scrape the sides and bottom of the bowl as needed. Using a rubber spatula, fold in 1 cup of M&M's, ½ cup of chocolate chips, and 1 cup of toffee pieces.

Step 8

Spoon the batter into the prepared pan, pressing into an even layer. Sprinkle the remaining 1/2 cup of toffee pieces, 1/4 cup of M&Ms, and 1/4 cup of chocolate chips over the top of the batter.

Step 9

Bake until a wooden pick inserted in the center comes out clean, 40 to 45 minutes.

Step 10

Let the cookie bars cool at least 30 minutes before slicing.

Step 11

Store the cookie bars in an airtight container at room temperature for up to 1 week.