Preheat your oven to 350°F. Line a 9×9 pan with parchment paper, spray with non stick spray, and set aside.
In a medium sized bowl, sift together the cornstarch, baking powder, salt, and flour.
In a large mixing bowl, use a whisk to combine the melted butter and brown sugar. Whisk by for 30 seconds.
Whisk in the egg and egg yolk, one at time for 30-45 seconds vigorously! This helps create that shine on top of the cookie. It will almost double in volume. Whisk in the vanilla.
With a rubber spatula, fold the dry ingredients into the wet ingredients, add the chocolate chips and M&M’s, and mix until no streaks of flour remain. Do not over mix.
Pour the batter into your prepared pan. Make sure to spread the dough evenly across the pan.
Add a few extra chocolate and M&M’s on the top (this is optional).
Bake for around 25-30 minutes or until the blondies are golden brown, set on the edges, and have puffed up a little. Mine took about 30 minutes, and my oven runs cooler. The middle will still be pretty doughy, but they will continue to cook as they cool! Let them cool for about 30 minutes.
Once they have cooled completely, top with flakey or regular salt, cut and enjoy!