If you find the spread is quite firm when making the donut mixture, warm it gently in the microwave before adding it to the other wet ingredients. This makes it so much easier to combine, it is worth the little extra effort!
Make sure to mix the wet and dry ingredients together only until combined, do not over mix. Over mixing the cake mixture will result in a tougher more gummy cake donut, not a light and fluffy one.
Nutella makes for a fabulous icing in itself! It is easily softened and spreadable, and sets to a perfect smooth finish for these donuts.
I like toasting my hazelnuts used for the topping. To toast them, simple place the whole nuts in a dry frying pan over medium heat. Keep moving them around the pan for about 10 minutes until they release a wonderful nutty aroma and are lightly golden in colour. Cool before chopping and using to decorate.