Line a large cookie sheet with parchment paper and preheat the oven to 300F.
In a large bowl, mix the dry ingredients- the flour, cocoa powder, sugar, cornstarch, and salt.
Add the melted butter and mix with a wooden spoon or nonstick spatula (you can also use a stand mixer fitted with the paddle attachment if you wish) to the wet ingredients in the large bowl
Mix with a good bit of effort, until the mixture starts to form large crumbs
Spread the crumbs onto the lined cookie sheet, breaking up the biggest ones into smaller crumbs. Bake for 20 minutes and make sure to break them any big clusters halfway through baking (about 10 minutes in). Remove from oven and let cool, they will get crunchy and hard as they cool! Set aside