Preheat the oven to 410°F. Line two baking sheets with parchment paper.
Add the cold butter, granulated sugar, and brown sugar to the bowl of a stand mixer. Mix until combined and crumbly. The mixture should not be fully creamed.
Add the eggs and vanilla extract. Mix just until incorporated.
Add the flour, cornstarch, baking soda, and salt. Mix until a thick dough forms.
Fold in the M&M’s, brownie brittle pieces, chocolate chunks, white chocolate chips, and sprinkles.
Divide the dough into large portions, about 5 to 6 ounces each. Shape each portion into a tall mound rather than a smooth ball to help create thick bakery-style cookies.
Press a few extra M&M’s, brownie brittle pieces, and chocolate chunks onto the tops of the cookies.
Place several inches apart on the prepared baking sheets and bake for 10 to 12 minutes, or until the edges are lightly golden and the centers still appear slightly underbaked.
Let the cookies cool on the baking sheet for at least 30 minutes. Allow them to finish setting for another hour before serving for the best texture.