1. Heat all ingredients in a small saucepan, stirring until it comes to a boil. Let it cool.
For the Cake:
1. Preheat the oven to 160°C.
Sift the flour and cocoa powder into a bowl, then add the sugar, baking soda, and baking powder.
2. In another bowl, whisk together the warm milk, eggs, melted butter, hazelnut butter, oil, and lemon juice. Using a whisk, combine the wet mixture with the flour mixture until well combined. Grease a 20x30 cm baking pan and pour in the mixture.
3. Bake for 30 to 35 minutes, or until a knife inserted comes out clean. As soon as the cake is out of the oven, drizzle it with the cold syrup.
For the Glaze:
1. Melt the chocolate in a bain-marie and spread it over the cold chocolate cake.