1. Preheat the oven to 180°C (350°F). Add the hazelnuts onto a baking sheet and toast them for about 10-12 minutes, or until golden brown and fragrant. Take the nuts out of the oven and let them cool down. Rub them with a kitchen towel to remove most of the skins, then chop half of them roughly and mix the other half in a mixer to get ground hazelnuts.

2. Add both sugars, water, oil, and vanilla extract to a large bowl and mix until well combined. In a second bowl, combine flour, baking powder, baking soda, espresso powder, and the ground hazelnuts. Add these dry ingredients to the bowl with the wet ingredients and mix until just combined. Add most of the roughly chopped hazelnuts and chocolate (keep some for the decoration) and fold in. Cover the bowl and place the dough in the fridge for at least 2 hours (or overnight).

3. Preheat the oven to 200°C (390°F). Line a baking sheet with baking parchment. Divide the dough into 12-13 portions and shape each one roughly into a ball. Place them with enough space in between on the baking sheet (keep the rest in the fridge) and decorate with the chopped nuts and chocolate you kept on the side. Sprinkle with some flaky sea salt (optional) and bake for about 12 minutes. Take out of the oven and let cool down on the baking sheet for some time, then transfer to a wire rack and let cool down completely. Repeat with remaining dough.