Make the Nutella: Preheat the oven to 350°F (175°C).
Spread the hazelnuts in a single layer on a baking sheet. Roast them in the oven for about 7-8 minutes, until slightly browned and the skins are blistered a little. Wrap the hazelnuts in a kitchen towel and rub vigorously to remove as much loose skin as possible. Let cool completely.
Melt the chocolate in the microwave or over a double boiler. Let cool slightly.
Place the hazelnuts in a food processor, grind the hazelnuts until they form a paste. Add the oil, sugar, cocoa powder, vanilla, and salt and continue to process until the mixture is as smooth as possible. Add melted chocolate and process until combined and smooth.
Strain the mixture to remove any chunks of hazelnut that remain (optional). Transfer to a jar or any other container.
Store the chocolate-hazelnut spread in an airtight container at room temperature for up to 2 weeks.
Make the brownies: Preheat oven to 350F (175C). Line the bottom of an 8-inch (20 cm) square baking pan with parchment paper, letting the paper hang over two sides.
In a large bowl mix Nutella and eggs, sift in flour, cocoa powder and salt. Stir until incorporated.
Pour into prepared dish and smooth top with spatula.
Bake for 17-18 minutes, or until a toothpick inserted comes out clean.
Let cool before cutting and serving.
Notes for making homemade Nutella:• The best option for the oil is hazelnut oil, if can’t find hazelnut oil you can use canola oil or any vegetable oil. • You can use more or less sugar, depending on your preference.