Preheat the oven to 350F degrees. Line a 9x13 inch pan with parchment paper and spray with non-stick cooking spray.
In a large bowl, whisk together the cooled, melted butter and sugar until mixed well.
Whisk in the eggs and vanilla extract.
Stir in the flour, cornstarch and salt just until mixed, with no flour showing through without overmixing.
Fold in the m&m's chocolate chips/ mix-ins.
Add batter into the prepared pan and smooth the top.
Bake in the preheated oven for 24-28 minutes, or until the top looks set and shiny. The edges will feel firm to the touch (not just set but actually kind of firm and crispy) and the surface will feel soft when pressed but will not spring back.
Cool fully before slicing, if it's possible. Excellent served warm with scoop of ice cream and fudge topping!