Preheat the oven to 175 degrees Celsius (350 degrees Fahrenheit) and line an 8 inch square baking dish with tin foil or parchment paper.
In a food processor add the hazelnuts and almond flour and blend until its all of a similar fine flour like texture. Transfer the mixture to a bowl and stir in the eggs, baking soda, vanilla extract, honey, ground coffee and melted coconut oil and mix until well combined
Pour the mixture into the lined baking dish. If topping with hazelnuts, evenly scatter them over the top of the batter.
Bake for 20 to 25 minutes until a toothpick inserted in the middle comes out clean.
If topping with icing in a food processor or blender combine all of the ingredients for the icing and blend until smooth, you may need to add a splash of almond milk or water to get it really smooth.
Once the coffee cake has completely cooled, evenly top with the icing and serve.