Cream butter and sugar using an electric mixer (about 5 minutes).
Beat in vanilla sugar, eggs (one at a time), salt and orange zest (leave a tablespoon aside for glaze); mix well.
Add milk and mix well.
Add flour and mix until fully incorporated, no clumps left. You should get a batter that is not too thick or watery. If you think it's too thick add some orange juice until you're happy with the texture.
Preheat the oven at 350 F = 180 C
Line a pound cake mold with baking paper.
Pour 2/3 of the mixture in then add a layer of Nutella (I poured 1/2 the mixture and Nutella ended at the bottom of the cake).
Add the second half of the batter.
Transfer the mold to the oven and bake on medium heat for about 30 min or until the cake is golden (test with a toothpick to see if it's done. If no batter sticks to the toothpick, then the cake is done). Don't open the oven on the first 10 minutes.
Allow the cake to cool in the mold then remove to a working surface.
To make the orange glaze whisk sugar with orange zest and orange juice in a small bowl until smooth. Add orange juice bit by bit until you're happy with the texture and consistency.
Pour glaze over the cake and sprinkle chocolate flakes (optional).