Preheat oven to 350 degrees F. Line a 9×13-inch baking pan with parchment paper and lightly grease. Set aside.

In a small bowl, whisk together flour, cornstarch, salt, baking soda, and baking powder. Set aside.

In the bowl of a stand mixer, cream together butter, brown sugar and granulated sugar until well combined. Scrape down the sides of the bowl with a spatula, as necessary.

Add the eggs and vanilla and beat until combined.

With the mixer off, add the dry ingredients. Mix on low until all the ingredients are justs combined. Don’t over mix.

Stir in 1 cup of M&M’s and the chocolate chips.

Press the cookie dough into the bottom of the prepared pan. Top with the remaining 1/4 cup of M&M’s. Sprinkle with flaky sea salt.

Bake for 20 to 24 minutes, or until the edges are set, and the top is just lightly browned.

Remove pan from oven and place on a wire cooling rack. Let the bars cool in the pan for until completely cooled before cutting. Use the parchment paper to lift the cookie bars out of the pan, then cut into squares and serve.