Grease an 8" square baking pan liberally with 1tsp of oil. Preheat the oven at 180°C for 10 minutes.
Add the all purpose flour, sugar, cocoa powder, salt, vanilla sugar, baking soda and and all spice powder(if using) to the pan and mix thoroughly.
Mix the coffee powder with the water and keep ready. Make two wells or depressions in the flour- 1 big and 1 small.
In the small depression pour the vinegar, and in the bigger one pour the oil. Now pour the coffee solution all over the pan and mix vigorously till everything is evenly incorporated.
Bake at 180°C for 30-40 minutes till a toothpick inserted comes out clean. Mine took 35 minutes. Once baked, cool the cake in the pan itself and frost with chocolate frosting.
The cake can be had plain or by just dusting some powdered sugar on top. But a chocolate frosting makes the cake even more delicious.
For frosting first mix up all the dry ingredients well, and then add the butter. Mix well till it becomes creamy. You can add a tbsp of milk if it becomes too thick, and more powdered sugar if it becomes thin. For this quantity a small cup and spoon are enough. But for large quantities using a hand mixer will be very helpful.