Make the Hazelnut frosting (Hazelnut Praline Crème Mousseline)

Start with preparing the pastry cream

• In a large enough bowl with the help of a Hand whisk, mix together sugar and egg yolk until slightly fluffy for about 1-2 minutes
• Mix in corn starch and flour until throughly incorporated and have a smooth paste
• Bring milk to simmering in a saucepan on medium heat. Make sure it is simmering but not crazy boiling
• Pour the warm milk over the egg yolk mixture slowly while whisking vigorously with a Hand whisk.
• Pour the mixture back into the saucepan and cook on medium heat for a few minutes until it thickens. Concretely it is approximately 1 min after the first boil. It might get lumpy first, don't worry just keep whisking and the cream will get smooth and glossy
• If in doubt, strain the pastry cream for a lump-free, creamy end result
• Add in room temperature soft butter and mix with a Rubber spatula until completely incorporated. It is worth adding the butter in 3 stages and mixing well after each addition
• Place the cream into a shallow bowl and cover the entire surface with plastic wrap to avoid skin forming on top. Let it cool to room temperature

Turn pastry cream into Hazelnut cake frosting

• Crème mousseline can be made with room temp pastry cream, room temp butter, and Hazelnut praline. Make sure everything is at room temp otherwise the mixture might curdle
• Whip room temp butter first for 1-2 minutes, then mix in Hazelnut Praline
• Start adding in the room temp pastry cream in 4 stages until the cream reaches a stable yet fluffy texture. Do not over whip the mixture as it can become runny
• Place the cream into the fridge for half an hour

Hazelnut Sponge

• Heat oven to 175 C / 347°F (no fan)
• Cream room temperature butter and sugar together in a stand mixer until pale and fluffy then whip the eggs in, one at a time
• Sift flour, hazelnut flour, salt, and baking powder together, this is the “dry ingredients” and the room temp milk is your wet ingredient
• Fold wet and dry ingredients with the butter-sugar mixture in the following order: wet, dry, wet, dry.
• Divide the cake batter between the 3 baking pans equally using a digital scale.
• Bake them at 175 C / 347°F (no fan) for 20 min or until a skewer inserted comes out clean. Let them cool on a cooling rack before frosting.

Assembling

• Start to build up the layers from the bottom to the top and fill the layers with 200g Hazelnut frosting each. Once the last cake layer is placed, chill the cake for a minimum of 30 minutes
• If at any point the frosting gets soft or the cake gets unstable, place it into the fridge to chill then only after continue.
• Crumb coat the cake with a thin layer of frosting and then chill the cake again for another 30 minutes before the final coating and decorating
• For the decoration, move the remaining hazelnut frosting into a piping bag fitted with a nozzle top and pipe decoration all around the top. Use some roasted chopped hazelnut to further decorate the cake
• Store refrigerated for max. 2-3 days