Line a baking tray with baking paper. Using two teaspoons dived the chocolate hazelnut spread into 1/2 teaspoon sized drops spaced out onto the parchment. Between 36-48 separate pieces will be fine.
Freeze the tray of chocolate hazelnut spread drops for at least 45 minutes before you move on to step 3.
Place the butter in a medium saucepan. Melt the butter over a medium heat.
Continue to cook until the butter foams and turns a nutty golden brown. Swirl the pan often to prevent burning. This will take around 3-4 minutes.
Weigh out 95g of the browned butter into a large mixing bowl. Re-weighing the butter accounts for variable water loss during browning. Set aside to cool, around 15 minutes.
Lightly toast your hazelnuts in a small frying pan over a medium heat until lightly golden. Around 2-3 minutes. Once cooled roughly chop the hazelnuts.
Once the butter is cool add in both sugars to the mixing bowl. Whisk until just combined.
Add the egg and vanilla and whisk for 2 minutes, or until the mixture has become lighter and thicker.
Add the flour, salt, baking powder and bicarbonate of soda. Fold the mixture together with a rubber spatula until almost fully combined. Do not overwork the dough at this point.
Add in the hazelnuts and frozen chocolate hazelnut spread balls. Fold through evenly. The chocolate hazelnut spread will melt slightly and form streaks through the dough, this is what you want.
Line a baking tray with baking paper. Use an ice cream scoop to make 12 equal sized balls. They will weigh around 56g each. Chill in the fridge for at least 30 minutes.
Take the cookie dough balls out of the fridge. Press the chocolate chunks onto each of the the cookies and re-roll into balls.
Chill for another 15 minutes whilst you preheat the oven to 190℃/170℃ fan (375℉/340℉).
Divide the dough balls between 2-3 lined baking trays. Allow around 3 inches/7.5cm between the cookies.
Bake for 11-13 minutes until lightly golden brown. Leave to harden on the trays for a couple of minutes
Remove from the oven. Whilst they are still warm you can use a large round cookie cutter to mould them into circular shapes, if they have spread unevenly. Sprinkle with flaky sea salt, if desired.
Leave to harden for a few minutes on the baking trays before transferring to a wire rack to cool completely.
