Mix flour, vanillin, baking soda and salt. In a large bowl, cream the butter with the brown sugar using an electric mixer. Add the egg, incorporating it well always with the electric whisk. Finally, add the flour mixture, mixing until you have a homogeneous mixture. Add the chocolate chips. 

With slightly moistened hands, form balls with a diameter of two, three centimeters, and place them on a baking sheet lined with baking paper, keeping them a couple of centimeters apart. Place the pan in the refrigerator for at least an hour. 

After the rest time, heat the oven to 180° and bake your soft biscuits for ten minutes. Take them out of the oven, leave them in the pan for five minutes, then gently transfer them onto a wire rack (they will still be very soft), letting them cool completely (as they cool they will reach the right consistency).