Whisk together the flour, baking soda, and salt in a bowl and set aside.
Melt butter in a saucepan over medium heat. The butter will begin to foam and make a crackling sound. Make sure you whisk constantly during this process. After a couple of minutes, the butter will begin to brown on the bottom of the saucepan; continue to whisk and remove from heat as soon as the butter begins to brown and give off a nutty aroma. Immediately transfer the butter to a bowl to prevent further cooking/burning. Set aside to cool for a few minutes.
With an electric mixer, mix the butter and sugars until thoroughly blended. Beat in the egg, yolk, vanilla, and yogurt until combined. Add the dry ingredients slowly and beat on low-speed just until combined. Gently fold in all the chocolate chips.
Chill the dough for at least an hour (2 hours is better) in the refrigerator.
Once dough is chilled measure about 1 1/2 tablespoons of dough and roll into a ball (I suggest using an ice cream scoop). Flatten the dough ball very thinly into the palm of your hand. Place 1 teaspoon of chilled Nutella in the middle and fold dough around it; gently roll into a ball. Make sure that the Nutella is not seeping out of the dough. Add more dough if necessary. Place dough balls on cookie sheet, seam side down, 2 inches apart.
Bake the cookies about 12-15 minutes or until the edges of the cookies begin to turn golden brown. They will look a bit underdone in the middle but will continue to cook once out of the oven. Cool the cookies on the sheets at least 2 minutes. Sprinkle with a little sea salt.
Transfer to a wire rack to cool completely.
Repeat with remaining dough.
Notes
DO NOT put in the freezer to speed up the process; it will be rock hard and nearly impossible to work with. DO NOT make the dough ahead of time and leave in the fridge overnight (See previous note about the freezer). These cookies take time and commitment, but they are worth every bit of it.I find it helpful to put the Nutella in a piping bag.For our Metric Friends - here are the conversions courtesy of Lindsey in New Zealand!!Preheat oven to 160C Ingredients: 285g plain flour 4.25g baking soda (bicarbonate) 1g salt 230g butter (unsalted) 245g brown sugar 50g granulated white sugar 1 whole egg plus one egg yolk 10mL vanilla essence 20mL plain or vanilla yoghurt 3 types of chocolate chips, any kind, totaling 300g 300g Nutella (chilled) Coarse salt for sprinkling