Preheat oven to 180°C (160°C fan-forced). Grease and line a 22cm spring-form cake tin with baking paper.
Place water, sugar or stevia, rice malt syrup or honey, butter or coconut oil, cacao powder and bicarbonate of soda in a large saucepan over low heat. Heat, stirring, until butter melts. Increase heat to medium and bring to a simmer – the mixture will rise and double in size. Remove from heat and allow to cool to room temperature.
Once cooled, add vanilla and whisk in eggs. Sift flour and baking powder into a bowl and add to egg mixture, whisking until just combined. Pour batter into prepared tin and bake for 45 minutes, or until a skewer inserted in the middle comes out clean. Remove from oven, place on a wire rack and allow to cool in the tin.
To prepare the icing, combine hazelnut butter with melted chocolate and half of the crushed waffle cone or buckinis. Mix well. Scoop the icing onto the cooled cake, spread gently over the top and top with hazelnuts and crushed waffle cone or buckinis before serving.
Store cake in the fridge, covered, for up to 7 days. You can freeze any remaining slices for up to 1 month.