Melt the butter in the microwave and stir in the cocoa powder until smooth.
Add the egg, cocoa mixture, milk, flour, sugar, vanilla (if using), and salt to a blender. Blend until smooth, about 10 seconds.
Let the batter rest for 10–30 minutes.
Heat a pan over medium heat and brush lightly with butter.
Once the butter is gently foaming, pour in about ⅓ cup of batter, swirling quickly to coat the bottom. Drop the heat to medium-low.
Cook until the bottom is set and the edges lift easily, about 1 minute. Flip and cook the other side for another minute. Re-butter the pan lightly between crepes.
Fold, roll, or serve flat with your choice of toppings.