Lightly grease a half-sheet pan.

Combine water, golden syrup or corn syrup, sugar, salt, and prepared hazelnuts in a 3-quart saucier over medium heat.

Scrape and stir until sugar has dissolved and the candy begins to bubble around the edges, about 5 minutes.

Adjust heat if needed to maintain consistent bubbling.

Continue cooking and stirring until the candy darkens to a tawny amber or foxy brown, registering between 320 and 340°F (160 and 171°C) on a clip-on digital thermometer, about 10 to 12 minutes more.

For delicate caramel notes, stop at the lower end of the temperature range; for a bolder, bittersweet flavor, aim for the higher end.

Remove from heat.

Immediately stir in baking soda, folding until the candy is evenly foamy.

Pour onto the prepared sheet pan.

Cool to room temperature.

Break the brittle into pieces.

Transfer to the bowl of a food processor.

Pulse several times to pulverize the candy.

Let the food processor run continuously until the brittle transforms into a thick, peanut butter-like paste, about 3 to 5 minutes.

Add cocoa powder and process until well distributed.

With the machine running, pour in melted chocolate, along with roasted-hazelnut oil.

Scrape the blade and bowl, particularly around the bottom and in the corners.

Continue processing until homogeneous and relatively smooth.

Transfer to an airtight container.

Store up to 1 month at cool room temperature or several months in the fridge.

The mixture will firm up to a truffle-like consistency after 24-36 hours.