Preheat oven
Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
Mix wet ingredients
Whisk melted butter, granulated sugar, and brown sugar until smooth. Add egg and vanilla and mix well.
Add chocolate
Stir melted chocolate into the wet mixture until fully combined.
Mix dry ingredients
In a separate bowl, whisk flour, cocoa powder, baking soda, and salt.
Combine
Gradually mix dry ingredients into wet ingredients just until combined.
Chill (optional)
Chill dough for 30 minutes for thicker cookies.
Bake
Scoop dough onto baking sheet and bake for 9–11 minutes, until edges are set and centers are soft.
Cool
Let cookies cool on the baking sheet for 5 minutes before transferring to a rack.