Place brownie mix in a large bowl. Add flour and baking soda and whisk. Add melted butter, egg, and vanilla to the dry ingredients. Use a hand mixer to mix until smooth.

Mix in the chocolate chips and nuts, if using.

Scoop 2 tablespoon sized balls of cookie dough on to parchment or silpat lined cookie sheets. Chill the cookie sheets for 1 hour.

Preheat oven to 350°F.

Bake chilled cookies for 8-10 minutes, until they just loose their glossy sheen. 8 minutes will be a little under baked if you like them that way, 9-10 will make the cookie more done.