In a saucepan, melt the butter over low heat. Strain the butter through a sieve to remove the scum then leave to cool for the rest.
On a baking sheet lined with parchment paper, place small, well-spaced heaps of Nutella (about 1,5 teaspoons - or the equivalent in a piping bag). Place the plate in the freezer for about 20 minutes.
In a bowl, beat the brown sugar, powdered sugar and warm melted butter. Then add the egg and vanilla and beat again.
Separately, mix the flour, baking powder, baking soda and salt. Add in 3 times to the cookie dough, mixing between each addition (with a fork, a spatula or by hand). Do not mix more than necessary to ensure a good final texture.
Stir in the chocolate chips last, keeping 1 tbsp of chips for decoration. Refrigerate the cookie dough for 15 minutes.
Preheat the oven to 190° traditional heat. Line a baking sheet with parchment paper. With an ice cream scoop, scoop out a portion of cookie dough then make a well in the center with your fingers. Add a dose of frozen Nutella inside then close the cookie ball by folding the dough all around.
Place the balls obtained on the baking sheet, spacing them well (I put 8 per sheet). Add the nuggets set aside for decoration on each ball.
(Put the plate with the Nutella back in the freezer / the dough in the fridge while you make the different batches).
Bake for about 15-16 minutes (this varies depending on the oven), the edges should be lightly browned. Note: the more you cook, the less tender they will be. Remove from the oven, let cool on a baking sheet then transfer to a wire rack.
Notes
These cookies will keep for about 4 days at room temperature in an airtight container. To warm them up and find their fondant inside, put them in the microwave for a few seconds.