For the brownies
- Preheat the oven to 180ο C (356ο F) set to fan.
- Melt the butter with the chocolate on a bain-marie, remove them from the heat, and set them aside at room temperature.
- In a large bowl, mix -with a hand whisk- the sugar, eggs, vanilla extract, cinnamon, and salt, until the sugar is dissolved.
- In a second bowl, add the flour, cocoa powder, the chocolate cut into pieces, and mix until the flour is combined with the chocolate.
- Mix, with a spatula, the second egg mixture along with the first mixture with the melted butter and chocolate, and add the third mixture with the solid ingredients.
- Butter and flour 3 cake pans of 20 cm each, divide the mixture into them, and bake for 20-25 minutes.
- Let the brownies cool onto a rack.
For the chocolate hazelnut frosting
- In a large bowl add the butter, icing sugar, the chocolate hazelnut spread, vanilla extract, milk, and beat well with a hand mixer until the mixture is fluffy.
- Place the first brownie onto a serving platter and spread ⅓ of the chocolate hazelnut frosting with a spatula.
- Cover with the second brownie and follow the same process for the third one too, by spreading the frosting over each brownie so to cover the sides of the cake with the last batch.
- Refrigerate the cake for 4-6 hours until it is chilled.
- Remove from the refrigerator and pour the chocolate hazelnut spread starting from the center and moving around so that it drips to the sides of the cake.
- Sprinkle with the toasted hazelnuts and serve.