In the bowl of an electric mixer, add the melted butter and brown sugar. Mix to combine, then add the eggs and vanilla. Mix on low just until combined.
In a separate large bowl, add the cocoa powder, baking soda, salt, and flour. Whisk, then add the flour mixture to the wet ingredients. Mix on low speed just until combined.
Remove the cookie dough from the fridge and use an ice cream scoop to scoop approximately 1½ tablespoon dough balls onto a cookie sheet about 2" apart.