Step 1

Preheat oven to 350°F. Line a large baking sheet with parchment paper.

Step 2

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.

Step 3

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!