In a medium skillet, on low-medium heat, add maple syrup and the larger pieces of chopped hazelnuts. Simmer for about 15 minutes until the maple syrup has thickened and the hazelnuts have crystalized. Transfer to parchment paper to cool.
Melt chocolate in a heavy sauce pot or double-boiler and set aside to cool.
In a large mixing bowl, cream butter until light and fluffy. Then add vanilla and powdered sugar, beat until well incorporated. If powdered sugar is clumpy, sift before adding.
Beat eggs in a small bowl until frothy. Then add the melted chocolate to the egg mixture, and beat until the chocolate just until well combined. Then add to the butter mixture and beat until there is no butter pieces visible and the mixture is smooth and creamy, about 4-5 minutes. Set frosting into the refrigerator to cool.
Cake:
Preheat oven to 350 °F. Prepare a 12 x 17 inch pan or six 8 inch round layer cake pans with removable bottoms with parchment paper.
Sift flour over parchment paper and set aside. In a large medium mixing bowl, beat egg yolks and ¼ cup sugar until very thick and lemon coloured, about 4-5 minutes. Set aside.
In a large mixing bowl, beat egg whites until frothy, and gradually add sugar beating well. Beat until rounded peaks are formed, about 3 minutes. Gently spread egg yolk mixture over beaten egg whites, folding in.
Divide the sifted flour into four portions. Sift one portion at a time over the egg mixture and gently fold just until blended after each addition. Transfer the batter into the baking pan or cake pans equal amounts and spread to ¼ inch thickness.
Bake for about 15 minutes, until lightly golden in colour. Remove to cooling racks and allow to rest for 4-5 minutes. Then carefully and quickly remove the parchment paper. Place the cake on the cooling racks to cool with right side up.
Once the cake is cooled, if using a sheet pan, cut cake into 6 equal pieces, first in half then divide each half into 3 sections. Remove the frosting from the refrigerator, and beat until fluffy.
Place the first layer on your serving tray, and begin layering. Add about 1/8-1/4 inch of the frosting after each cake layer is added. Once the cake is frosted, add the maple glazed hazelnuts to the top and bottom sides of the cake.
Refrigerate until ready to serve.
Notes
Store in the refrigerator for 1-2 days. Freeze up to a month in a freezer safe container.