Preheat oven to 350°F (175°C). Whisk flour, cocoa powder, baking soda, and salt in a medium bowl.

In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth. Whisk in egg, vanilla, and oil until glossy.

Stir the dry mixture into the wet until no flour streaks remain. The dough will be soft and slightly sticky. Chill briefly if needed.

Scoop 1 1/2 tablespoon portions of dough, roll into balls, then coat heavily in the coarse sugar. Place 2 inches apart on lined baking sheets.

Bake 9 to 11 minutes until edges are set and centers are puffed. Let rest on the sheet for 5 minutes, then transfer to a wire rack to cool completely.