Preheat your oven to 350°F. Grease and line a 9x13 metal pan with parchment paper.

In the bowl of a stand mixer, combine the melted butter, brown sugar, and granulated sugar. Mix until everything is well combined.

Add in the vanilla extract and eggs, and mix until fully incorporated.

With the mixer on low, gradually add in the flour, salt, cornstarch, and baking soda. Mix until just combined.

Gently fold in the M&Ms and sprinkles using a spatula.

Use the spatula to evenly spread the batter into the prepared pan.

Bake for 22-25 minutes or until the edges turn golden brown.

Once done, remove from the oven and allow the bars to cool completely before cutting into delicious squares.