Chocolate Hazelnut Cake

1. Preheat the oven to 325F/165C.
2. Lightly grease a round 8 in/20 cm (the pan should be at least 3 inch/7.5 cm deep) baking pan with a little oil and line with parchment paper.
3. In a large bowl, dry whisk the flour, cocoa, baking powder, and salt. Make a well in the center.
4. Pour the boiling water over the chocolate chips in a small bowl and stir with a silicone spatula or whisk until the chocolate melts and is smooth and silky. Pour the melted chocolate into a blender, add the oil, sugar, eggs, sour cream, coffee, and hazelnut extract, and pulse at high speed for a few seconds at intervals until the mixture is combined and emulsified. Don’t overdo the blending, or your cake might turn dry.
5. Pour the liquid from the blender into the center of the flour mixture. Stir with a wooden spoon, a spatula, or a whisk until combined and smooth and there are no visible flecks of dry flour. Pour the mixture into the prepared baking pan, level the surface with an offset spatula, and bake in the preheated oven until a skewer or knife, inserted through the cake’s center, comes out clean, 45 to 50 minutes. Let the cake cool in the pan for 5 minutes, then run a knife along the edges to help release the cake and transfer it to a wire rack to cool completely. The cake can be made a day or two ahead of time and stored wrapped with clingfilm in the refrigerator or for a month in the freezer in an airtight bag.

Hazelnut Praline

1. Wet a baking sheet with a little water and then line it with parchment paper, this will help prevent the paper from moving when the pan is rotated. Add the sugar to a medium saucepan or saucier. Heat over medium-low until it reaches 320F/160C, 15 to 20 minutes. Stir occasionally using a silicone spatula. The sugar will melt and start turning a light brown caramel shade.
2. Fold in the hazelnuts and the salt flakes, stirring for 5 minutes until the hazelnuts turn fragrant. Pour the hot mixture straight onto the lined baking sheet and carefully tilt and rotate the pan to help the hot mixture spread in a single layer. Leave aside to cool completely and harden. Break into large shards; reserve half for the cake topping and the rest for the frosting. The whole praline can be made one day in advance and stored in an airtight bag in a cool part of the kitchen. Because it can stick easily, I prefer storing the whole sheet of praline versus cracking it. 

Chocolate Buttercream Frosting

1. Add the butter and confectioner’s sugar to the bowl of a food processor fitted with a blade—pulse for a few seconds until combined; the butter should be soft and fluffy. Scrape the sides of the bowl down. Add the remaining ingredients for the frosting and pulse for a few seconds until combined. Scrape the sides of the bowl down often. The chocolate chips should leave tiny flecks in the frosting. Transfer the frosting to a bowl. The frosting can be made a day ahead of time. Transfer and store in an airtight container; cover the surface with a sheet of clingfilm or parchment paper, pressing down gently. Refrigerate until ready to use. Before use, soften the frosting by leaving it out on the kitchen counter for at least 30 minutes until pliable. 
2.  When ready to use, take 2 cups of buttercream frosting in a large mixing bowl. Reserve the remaining buttercream frosting to coat the outside of the cake. Take half of the hazelnut praline and grind it to a coarse or fine powder in a food processor or blender, depending on how grainy you want the frosting. Fold until combined and keep aside until ready to use. This can be done a day ahead of time. Store in an airtight container until ready to use. Before use, soften the frosting by leaving it out on the kitchen counter for at least 30 minutes until pliable.

Assembly

1. You can trim the top and edges of the cake with a sharp, serrated knife (I prefer trimming the tops off). Use the cake only when completely cooled (preferably chilled); otherwise, the buttercream will melt. Cut the cake in half through the center. 
2. Place a cake board or round on a turntable and place a tablespoon or two of buttercream frosting in the center. Center one cake half on the board; the buttercream will help glue the cake while frosting. Using an offset spatula, spread the buttercream mixed with the chocolate hazelnut praline in the center of the cake. Place and center the second cake on top. Refrigerate the cake for 30 minutes (I prefer this because it firms up the buttercream and holds everything together nicely.)
3. Coat the top and the sides with 1 cup of buttercream using an offset spatula to form a thin layer. Transfer the cake to the refrigerator and allow this “crumb coat” layer to firm up for about 30 minutes. Once this layer of buttercream has hardened, remove the cake from the refrigerator and coat it with the remaining buttercream using the offset spatula. Rotate the turntable while spreading the buttercream to get a smooth and even layer on the sides and the top of the cake. Garnish the top of the cake with the remaining shards of the hazelnut praline, and serve the extra on the side with the cake. The cake can be served immediately or stored covered for up to 3 to 4 days at room temperature or refrigerated for up to 1 week in an airtight container.