In a bowl, whisk together the eggs, milk, sparkling water, flour, vanilla, salt, and granulated sugar until smooth. You can also use a blender for a quicker mix. Rest the batter for about thirty minutes in the fridge to let the flavors meld and the bubbles settle.
Place a crepe skillet over medium heat. Lightly brush the skillet with some vegetable oil or melted butter to prevent sticking. Once hot, pour in about a quarter cup (sixty ml) of the batter, tilting the skillet to coat the bottom evenly.
Cook the crepe for about thirty to forty seconds until the edges look dry and the bottom is lightly golden. Carefully flip it and cook for an additional thirty to forty seconds on the other side. Repeat this process until you’ve used up all the batter, yielding around thirty to thirty-five crepes.
In a separate bowl, cream together the mascarpone, honey, and vanilla until smooth and fluffy. This will be the delicious filling between the layers.
Set the mascarpone mixture aside while you prepare the hazelnut brittle. Line a baking tray with parchment paper and lightly brush it with vegetable oil. Roughly chop the hazelnuts.
In a medium saucepan over low-medium heat, add the granulated sugar and melt it until it turns caramelized, stirring constantly to prevent burning. Once melted, mix in the chopped hazelnuts and pour the mixture onto the prepared tray. Spread it evenly and let it cool completely as it hardens quickly.
To assemble the cake, place one crepe on the serving platter. Spread about two to three tablespoons of berry jam over it, leaving a half-centimeter (one-fifth inch) border around the edges.
Top with another crepe, and this time spread two to three tablespoons of the mascarpone frosting on it, again leaving the same border. Repeat this process, alternating layers of jam and mascarpone until all crepes are used. Do not add filling to the top crepe.
If your cake has a dome shape from stacking, gently press down in the center to flatten it out. Wrap the whole cake in cling film and refrigerate for at least four hours to allow it to set.
Once you’re ready to serve, remove the cake from the fridge. Spread the remaining mascarpone frosting over the top and garnish with fresh berries and pieces of the cooled hazelnut brittle. Enjoy your masterpiece!