• Meringues: Heat oven to 225°F (110°C). Line 2 baking sheets with parchment and draw three 8 inch circles on the parchment using an 8 inch plate or the bottom of a removable tart pan. Butter or spray the parchment.
• In a food processor, grind hazelnuts until they are powdery but not a paste. Put into a medium bowl. Do the same with the almonds. Add to same bowl. Sift in confectioners' sugar and combine with the nuts. Add salt to bowl and mix well.
• Put egg whites into the bowl of a stand mixer. Whisk on medium-low for 2 minutes.
• Increase speed to medium and whisk until soft peaks form, another 2½ to 3 minutes. Gradually add in the granulated sugar and continue whisking at medium-high until medium-stiff peaks, another 1½ minutes. Remove bowl from stand mixer and gently fold the nut mixture into the egg whites ⅓ at a time with a rubber spatula. Divide egg white mixture between the three 8 inch circles on the parchment paper. Spread from middle-out. Bake in middle of oven until the circles are firm to the touch, 3 to 4 hours. Rotate pans hourly. Turn oven off and leave the circles in the closed oven for 6 hours or up-to 12 hours to slowly cool.
• Assemble: Place one dacquoise on a flat cake plate. Carefully spread ½ the ganache (recipe below) evenly in a ½-inch layer over the bottom dacquoise. Press second dacquoise gently over the ganache layer to adhere. Add an equal layer of buttercream (recipe below) over that. Place the last dacquoise flat-side up and gently press it down on to the buttercream layer. With an offset spatula, spread remaining butter cream around the sides of the cake to fill in gaps and smooth the sides out. Smooth any remaining buttercream in a thin layer over the top. Place the cake in the refrigerator for at least 1 hour before serving and up-to 2 days wrapped well. Cake can be made to this point 2 weeks in advance and frozen. Defrost overnight to use the next day.
• Finish: Melt remaining ganache in a microwave for 45 seconds. Place cake on a wire rack over a baking tray. Pour the ganache over the top of the cake and let it drip over the sides. With an offset spatula, smooth top and sides so that the dacquoise is completely covered in smooth dark chocolate. Press all the sliced almonds all around the sides of the cake. Decorate the top with whole hazelnuts. Serve sliced or cake may be placed in the refrigerator in an airtight container for 2 days before serving.

Expresso Buttercream

• Place water and sugar in a small saucepan. Gently stir to combine. Over high heat, cook until thickened and around 240° on a candy thermometer, 3 to 4 minutes.
• In the bowl of a stand mixer, whisk eggs and egg yolk on medium speed until pale and light, about 3 to 4 minutes. When water-sugar syrup reaches temp, carefully drizzle syrup down side of bowl still on medium speed. Whip another 7-8 minutes until mixture becomes fluffy. Add the softened butter in pieces and continue whipping for another 4-5 minutes. Add espresso powder and salt, and whip to combine. Use right away or store in airtight container for up to 5 days in the refrigerator. To use, remove from refrigerator and bring to room temperature. Use a stand mixer with paddle attachment to bring it back to a creamy state.

Ganache

• Chop the chocolate and put in a bowl. Heat the cream in a small saucepan until just before it reaches a boil. Small bubbles will begin to collect along the side of the saucepan. Pour hot cream over the chocolate pieces in the bowl and let sit for 30 seconds. Slowly whisk them together until they are combined and smooth. Put aside in an airtight container at room temperature or put in refrigerator for up to a week. Place on the counter a day before using.