Put the oven racks in the top two positions.
In your mixing bowl, mix the softened butter, brown sugar, and granulated sugar until creamy.
Add the vanilla and eggs and mix thoroughly.
Add the flour, baking soda, and salt and mix.
Slowly add the rolled oats and stir until blended. The mixture will be very thick, be careful not to overwork your blender.
Using the large 3 tablespoon cookie scoop, scoop out cookie balls and place them on a lined baking sheet.
Remove the Nutella balls from the fridge and place them beside your cookie balls.
One at a time, press a hole in each cookie ball with your thumb and insert a Nutella ball in each cookie then close the cookie ball around the Nutella and form it into a sphere again. Place them on a lined baking sheet spaced a few inches apart.
Sprinkle some fleur de sel on each cookie.
Bake for 12 minutes.
If you are using two cookie sheets, switch them halfway.
Bake until the edges are slightly browned but the center is still light: this will ensure a soft and chewy cookie,
After the cookies are cool enough, transfer them to a wire rack to cool fully.