Grease a 7'' round cake tin and dust it with flour. Keep it aside.
Toast hazelnuts and grind coarsely. Toasting enhances the flavor. Toast in oven or a wok till light golden.
Start preheating oven at 180 degrees for C for 15 minutes. If baking in microwave convection then preheat at 170 degrees celsius.
Sift together wheat flour, cornflour, cocoa powder, baking soda, instant coffee powder, and salt at least 3 to 4 times.
Mix ground hazelnuts and brown sugar in it.
Make a well in the centre and add oil, vanilla essence, Nutella, water and vinegar in it.
Mix everything lightly using cut and fold method. Do not mix it vigorously else the cake will turn out hard.
The batter should be of flowing consistency. If need be, adjust the consistency of the batter by adding little more water.
Pour the batter in the prepared cake tin. Tap it lightly 2 to 3 times to release air bubbles, if any.
Bake in the preheated oven for 30 to 35 minutes.
Check if the cake is baked fully. Insert a toothpick and see that it comes out clean. Along with this, press the cake lightly and it should feel spongy. If not, bake for few more minutes.
Take out the cake tin and wait for 8 to 10 minutes till it cools down slightly.
Invert it on a wire rack.
Let it cool completely, then, wrap it in cling film and keep it in the refrigerator for 4 to 6 hours. If possible, keep it overnight.