1. In a bowl, sift together flour and baking soda. Set aside. Pre-heat the oven 170℃.
2.Start with the Nutella filling Use a teaspoon and place 10 portions of Nutella on the prepared baking sheet. Place them gently in the freezer for 20 to 30 minutes or until firm. As they get harder they will be easy to use.
3.Meanwhile make the cookie dough In a bowl, add the butter (softened), sugar (both brown and white), salt and mix well with a spatula until smooth.
4. Incorporate the egg little by little (to prevent curdling) and make into a smooth mixture.
5. Mix in the dry ingredients from #1 until combined with a spatula. Wrap the dough with a plastic wrap and let it sit about 30min in the fridge.
6. Divide the cookie dough into 10 pieces (about 15g each). Roll each one into a ball, then flatten it. Place one frozen nutella portion (from the freezer) in the middle of the flattened cookie and pull the edges together to wrap the nutella ball.
7. Place them on a cooking sheet. Bake for 9~10min until the cookies look puffy but the centers still look soft. Do not overbake.
8. Remove from the oven. At that point I like to immediately press lightly the cookies to create a sort of decoration. Let it cool on the trays for about 5 minutes, then transfer the cookies to a wire rack to cool down completely.