Make the Meringue

1. Preheat oven to 250°F and position rack in the centre.
2. Draw a 13×10½-inch rectangle on parchment paper, flip it over, and place it on a greased baking sheet.
3. In a food processor, chop the almonds, hazelnuts, cornstarch, salt, and half the sugar until fine.
4. In a stand mixer, beat the egg whites and cream of tartar on medium-low until foamy (1 min), then medium-high to soft peaks (1 min). Slowly add the remaining sugar and beat to stiff, glossy peaks (2–3 min).
5. Gently fold in the nut mixture in two parts.
6. Spread the mixture onto the parchment within the rectangle. Mist lightly with water.
7. Bake for 1½ hours. Turn off the oven and let the meringue cool inside (do not open the door) for another 1½ hours.
8. Let cool at room temperature for 10 minutes. Wrap tightly in plastic if storing (up to 2 days).

Make the Buttercream

1. In a medium saucepan, heat the milk until just simmering.
2. In a medium bowl, whisk yolks, sugar, cornstarch, and salt until smooth. Slowly whisk in half the hot milk to temper.
3. Pour the yolk mixture into the remaining milk in the pan. Cook over medium heat, whisking, until thick like warm pudding (3–5 min).
4. Transfer to a bowl, cover, and chill for at least 2 hours (or up to 24). Before using, gently warm to room temperature in the microwave.
5. In a small bowl, mix the espresso powder and water.
6. In the bowl of a stand mixer, beat the butter until light and smooth (3–4 min). Add pastry cream in 3 parts, beating 30 seconds each time. Add espresso and beat until fluffy (5 min), scraping bowl midway.

Make the Ganache

1. Put the chocolate in a heatproof bowl.
2. Heat cream and corn syrup in a saucepan until simmering. Pour over chocolate; let sit 1 min, then stir until smooth.
3. Let cool until it slightly mounds when spooned (about 5 min).

Assemble the Dacquoise

1. Carefully peel the parchment off the meringue. Using a serrated knife, trim to 12×10 inches. Carefully cut into four 10×3-inch strips. Using an offset spatula, spread ¼ cup of ganache on each of the three strips. Chill until set (15 min).
2. Spread ½ cup buttercream on the uncoated strip. Stack with the ganache-coated layers, ganache side down, spreading ½ cup buttercream between each.
3. Coat the sides of the stack with half of the remaining buttercream; spread the rest on top. Chill for 2 hours. Wrap and refrigerate if storing (up to 2 days).

Final Touches

1. Rewarm the ganache gently until fluid but not hot. Pour over the chilled cake, spreading evenly over the top and sides.
2. Garnish the top with the 12 hazelnuts in a line down the centre.
3. Press sliced almonds onto the sides. Don’t worry if some ganache shows through.
4. Chill uncovered for 3–12 hours. Transfer to a platter.
5. Slice with a sharp knife dipped in hot water and wiped dry between cuts.