Preparation

Prepare the baking sheets:

• On two large sheets of parchment, use an 8-inch template (a tart pan or a cake circle is good) to trace two circles on each sheet, 1 inch apart. Dab the corners of two baking sheets with butter. Press each sheet of parchment, marked side down, onto a baking sheet to make it stick. Lightly butter and flour the parchment.
  

Grind the hazelnuts and sugar:

• In a food processor, pulse the 5 oz. of hazelnuts with the granulated sugar to a fine, dry meal. Measure 1 cup; reserve the rest for garnishing the cake. Position the oven racks to divide the oven into thirds and heat the oven to 275°F
• Sift together 3/4 cup of the superfine sugar and the cornstarch into a bowl. Stir in the cup of ground nut mixture and set aside.

Whip the whites into a meringue:

• Combine the egg whites with the cream of tartar and salt; whip on medium speed until foamy.
• Increase the speed to medium high and gradually add 1/2 cup superfine sugar. Whip until glossy and smooth and medium-stiff peaks form. During the last few turns to the whisk, add the vanilla and turn the speed to high.
  

• In four or five additions, fold in the nut-sugar mixture, sprinkling and then folding gently so you don’t deflate the foam.

Pipe the dacquoise disks and bake until crisp but still pale:

• 勺fitte dacquoise糊成一个糕点包d with a 1/2-inch plain round tip. Push the mixture toward the tip and twist the bag closed. Arrange the parchmentcovered sheets on your work surface.
• Mark the center of each circle with a dot of batter. Starting at the center, pipe the meringue in a continuous spiral until you touch the outer edge. Repeat with the remaining three circles. Also pipe a few 3-inch spiral cookies; you’ll use these to test for doneness.
• Smooth any gaps with a toothpick. Bake in the heated oven until the disks feel dry but are still quite pale. Check the oven after 20 minutes; if browning is apparent, reduce the heat to 250°F and continue baking for about 80 min. total.
  

• Test for doneness by breaking one cooled cookie in half. If it’s evenly crisp throughout, the disks are ready. If it’s still a little soft inside, bake the disks about 10 min. longer without raising the heat. Slide the parchment onto racks. Cool the disks completely and then gently peel off the parchment.

Trim the cake layers:

• Set your tracing template gently on top of each disk. Without pressing down, cut around the edge with a sharp paring knife, shaving away uneven edges (reserve the trimmings). Store the layers between sheets of parchment in an airtight container until you assemble the cake. Crush the disk trimmings into coarse crumbs; add them to the reserved ground nuts.
  

Make the chocolate mousse:

• Melt the chocolate and the butter over simmering water. When most of the chocolate has melted, remove from the heat, stir until smooth, and set aside for about 5 minutes to cool to 100°F on an instant-read thermometer. Beat hard until perfectly smooth. Whisk in the four egg yolks one at a time. Set the mixture aside to cool.
  

• Beat the four egg whites with the salt until foamy. Gradually add the 3 Tbs. superfine sugar and beat until the whites are glossy and smooth and medium-stiff peaks form. The beater should mark tracks in the mixture.
• When the chocolate feels slightly cooler than body temperature, stir about a quarter of the whipped whites into the chocolate to lighten it. Fold this chocolate mixture into the remaining whites in three or four additions, laying the chocolate across the top, folding gently, and scraping the sides and bottom as needed.
• In a chilled bowl with chilled beaters, whip the cream until firm peaks form. During the last moment, add the vanilla. Fold the whipped cream into the cooled chocolate mixture; it’s fine if it’s slightly streaked when you’re done folding. Refrigerate, covered, at least 3 hours but no longer than 2 days.

Glaze the hazelnuts:

• Gently poke a pointy wooden toothpick into the bottom of each hazelnut at the point slightly off the center junction of the bottom lobes so the nut won’t split. Insert it just enough to grab or the nut will split. Prepare a shallow hot water bath.
  

• In a small, deep saucepan, combine the sugar, water, and cream of tartar. Set over medium heat and cook, swirling but not stirring, until the sugar is dissolved and the syrup is clear. Wash down the pan sides with a pastry brush dipped in cold water. Raise the heat to medium high and boil the syrup, swirling the pan but not stirring.
• Cook until the syrup turns light amber, about 10 minutes. Immediately remove the pot from the heat and set it into the water bath. Let sit until the bubbling subsides (not more than 15 minutes). Tilt the pan and carefully dip the nuts into the syrup, twirling as you remove them. Stick them upright in an apple or styrofoam block to dry, being careful not to drip the hot caramel on your fingers. If it’s humid, refrigerate the nuts and apples.
  

Assemble the layers and decorate the cake:

• Put a dab of mousse in the center of a foil-covered cardboard cake circle or a tart pan bottom. Center a dacquoise disk on top.
• 传播3/4杯奶油冻on the disk. Top with another disk and repeat with all the disks (ending with a disk), making sure the stack is straight. After using all the disks, refrigerate the cake and remaining mousse for about 15 minutes to firm up.
  

• Spread a cup of mousse around the cake sides and another cup over the top. If the cake starts to melt or slide as you frost it, return it to the refrigerator.
  

• Scoop up the nut-crumb mixture and gently press it onto the bottom of the cake sides, going two-thirds of the way up all around the cake.
  

• Fit a pastry bag with a 1/4-inch star tip, fill with 3/4 cup mousse, and pipe twelve 1/2-inch rosettes, spaced evenly, around the edge of the cake top. Refrigerate for at least an hour (or up to 7 hours).
  

• Just before serving, top each rosette with a candied hazelnut. Cut the cake with a sharp, thin knife.