Preheat oven to 350°F. Line a large baking sheet with parchment paper.
Beat together butter, brown sugar, vanilla and salt in a small bowl with a spatula until smooth and creamy. Mix in egg yolk and baking soda. Combine flour and cocoa to remove any lumps and then fold it in. Fold in chocolate.
Roll heaped tablespoons of dough into balls and place onto prepared baking sheet spacing them 2 inches apart. Flatten slightly. Bake for 8-10 minutes until puffed, set around the edges but still soft in the middle. Transfer to a wire rack to finish cooling, but definitely eat a few while they are warm!