Step 1

Heat the Oven – Set the oven rack to the middle position. Heat the oven to 350ºF (177ºC). Line a 13×9 metal cake pan with foil or parchment paper. Leave some overhang on the sides to make the cookie easy to lift out. Lightly grease the pan with nonstick cooking spray.

Step 2

Brown the butter – Heat the butter slices in a 10-inch skillet over medium heat, swirling to melt evenly. Stir and cook, scraping the bottom of the pan with a spatula, until butter is golden brown and nutty in aroma, 2 to 3 minutes. Immediately transfer to a large bowl, scraping all of the bits from the pan. Cool to below 144ºF (62ºC).

Step 3

Mix the Dry Ingredients – In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

Step 4

Mix the Sugar and Eggs – To the cooled browned butter, add the brown sugar. Whisk until combined. Add the eggs and vanilla. Whisk until smooth and shiny.

Step 5

Make the Dough – Add the flour mixture to the butter mixture. Fold together until no flour streaks remain. Do not over mix!

Step 6

Add the Mix-Ins' – Add the chocolate chips and M&M candies. Stir until combined in the dough.

Step 7

Bake – Evenly spread the dough in the prepared baking pan. Press some chocolate chips and M&M candies into the surface. Bake in the center of the oven until the surface is lightly golden brown and the edges are set, 20 to 22 minutes.

Step 8

Cool – Transfer the cookie bars to a wire rack to cool in the pan until it's room temperature. This can take about 1 hour. If removed when hot, it will break in the middle. To serve warm, slice in the pan and remove the pieces with a spatula.

Step 9

Slice – Carefully use the foil or paper overhang to lift the cookie out of the pan. Place on a cutting board, remove lining, and slice into 2-inch-sized pieces.