In a container, beat the soft butter, the brown sugar and the white powdered sugar. The mixture should be very creamy. Add egg and vanilla. Beat again.
Separately, mix the flour, baking soda and salt. Fold into the batter with a spatula, being careful not to over mix (just enough).
Finish by adding the oat flakes and large chocolate chips (keep 20g of chips for decoration).
Cover the container and refrigerate for 1 hour.
Preheat the oven to 180° traditional heat. Using an ice cream scoop or your hands, form balls of cookie dough then place them on the baking sheet covered with parchment paper, spacing them well (you will need to make 2 batches of 8 cookies)
Crush the balls of dough a little then add a few more nuggets on top.
Bake for about 12 minutes (depending on your oven). The edges should be lightly browned, the center not overcooked. Leave to cool on a plate, they will harden as they cool, while remaining soft in the middle.
Tip: when they come out of the oven, if your cookies do not have a very round shape, take a mini pastry circle, place it above the cookie and make circles, this will firm up its sides and give it a nice round shape. .