You won't need to preheat right away, since this dough will need to chill first. In a large bowl, whisk melted butter and cocoa powder.
Add brown sugar and granulated sugar; beat on medium speed until combined.
Add eggs and vanilla; beat until incorporated. Set aside.
In a separate bowl, whisk flour, baking soda, and salt. Stir into the wet mixture.
Cover and refrigerate 45 minutes.
Preheat oven to 350F. Roll dough into 1.5-in balls and place on ungreased baking sheets, 2 in apart.
Bake 8–10 minutes, until set.