Melt butter in a light‑colored skillet over medium heat. Once melted, stir regularly as it foams, and the milk solids turn deep golden brown, around 4-5 minutes. Pour into a heatproof bowl (it will have reduced to around 120g after being browned) and let cool at room temperature until opaque and soft‑solid, or chill in the fridge for 10–15 minutes, checking often to prevent it from getting too hard.150 g unsalted butter
While the butter cools, preheat your oven to 350°F (180°C).
Toasting hazelnuts: Spread the hazelnuts on a baking sheet and toast in the oven for 8–10 minutes, tossing halfway, until lightly golden and fragrant. Remove them from the oven and set aside. Line the baking sheet with a Slipat mat or lightly greased parchment paper. 100 g hazelnuts
In the light-colored pan add sugar in an even layer. Keep the heat at medium and let the sugar start to melt around the edges. As the edges liquefy, use a heatproof spatula to bring the unmelted sugar into the pool. Watch it carefully.100 g granulated sugar
Once the sugar is a deep golden amber and all melted tip the toasted hazelnuts into the caramel, stirring so that each nut gets a thin candy shell. Work quickly so the caramel doesn't burn. Scoop the nuts onto the lined sheet into a single layer. Let it cool down and harden.
Once cooled, chop the nuts into a mixture of fine pieces and slighly larger ones. You can also do this by giving them a few pulses in a food processor. Set aside.
When the butter is cooled and ready, add the sugars and vanilla. Using a hand mixer on medium speed, whip the mixture until pale and fluffy, about 2–3 minutes. 100 g soft brown sugar, 50 g granulated sugar, 1 tsp vanilla extract
Scrape down the bowl, then beat in the egg. 1 large egg
In a medium bowl, whisk together the dry ingredients, then add this dry mixture to the butter mixture until just combined. 190 g all-purpose flour, 3/4 tsp salt, ½ tsp baking soda, 1/4 tsp baking powder
Using a spatula or wooden spoon, gently fold in the chopped caramelized hazelnuts and chocolate chips until evenly distributed. 90 g semi-sweet chocolate chips or chunks
Roll the cookie dough into balls, around 1 1/2 tablespoons of dough for each. Space them out on the baking sheet, around 2 inches apart.
Bake one sheet at a time for around 10-11 minutes until edges are set and centers still look just slightly underbaked. Sprinkle with flaky sea salt. Let cookies rest on the pan for 5 minutes, then transfer to a wire rack to cool completely. Flaky salt