Preheat the oven to 350 F. Set the hazelnuts on a cookie sheet and toast for 10 minutes, shaking the pan around 5 minutes for even heating.

While still warm, put the hazelnuts in a food processor and begin grinding them. As needed, stop the machine and scrape down the ground nuts, keep going as you start to see a thick paste forming, then a looser paste, and after about 10 minutes or so, you’ll have a very loose soup like paste. Transfer to a mason jar.

Preheat the oven to 350 F. Line a 8 or 9” square pan with parchment paper.

Crack the eggs into a bowl (use a bowl and a hand mixer or a stand mixer) and add the sugar, immediately begin beating them (paddle attachment for a stand mixer) until it becomes very light and frothy. Add the oil while beating it and mix to combine.

Add the hazelnut butter, vanilla and salt and beat to combine.

Sift the cake flour over the batter, along with the baking powder. Turn the mixer on, and while it’s beating pour in the buttermilk. Mix until the cake batter is combined and shiny.

Pour into the prepared pan. Bake the cake for 30-40 minutes, it’s done when a cake tester comes out clean or when you can press the top and it springs back.

Let it cool before making the topping.

In a pot, add the butter and set over medium low heat. Once it’s melted, add the sugar and whisk to combine. Let it cook and come to a boil for about 2 minutes. Then add the milk and whisk. Cook the mixture for another 2 minutes without stirring, then remove from heat.

Add the hazelnut butter, salt and vanilla and stir to combine. Add the powdered sugar and whisk, vigorously, to break up all the sugar bits until you have a very smooth, shiny frosting (it won’t be thick - don’t worry).

Pour it over the cake in an even layer and leave it to set. Soon after you’ll notice the penuche has a matte top and is thickening. After about an hour (you can hasten this along if you set it in the fridge) it’ll set into a soft but stable layer.