Adjust oven rack to middle position and heat oven to 250 degrees. Using ruler and pencil, draw 13 by 10 ½-inch rectangle on piece of parchment paper. Grease baking sheet and place parchment on it, ink side down.
Process almonds, hazelnuts, cornstarch, and salt in food processor until nuts are finely ground. Add ½ cup sugar and pulse to combine.
Using stand mixer fitted with whisk, whip egg whites and cream of tartar until glossy, stiff peaks form.
Fold nut mixture into egg whites and spread meringue on parchment. Bake for 1 ½ hours.
Make pastry cream, chill, and beat with butter to make (coffee) buttercream.
Make ganache by combining chocolate, cream, and corn syrup. Let cool.
Assemble the dacquoise by layering meringue with buttercream and ganache. Refrigerate until firm.
Warm remaining ganache and pour over the top of the cake. Spread ganache and garnish with hazelnuts and almonds.
Chill the dacquoise for at least 3 hours before serving.
Cut into slices and serve.
