For Meringue Japonais:.
In food processor fitted with steel blade, combine 3/4 cup nuts and 3 tablespoons sugar (reserve remaining sugar). Process by "pulsing" on-and-off just until nuts are finely ground. Place in a small bowl. Add cornstarch and mix in thoroughly with fingers. Cover and set aside.
Adjust racks to divide oven into thirds; preheat oven to 225 degrees F. Line two large (17" by 12") baking sheets with parchment paper. On each piece of parchment, trace a 10" diameter circle. Set aside.
Make sure that large bowl of electric mixer is absolutely clean and grease-free; place room-temperature egg whites into this bowl. Fit mixer with whisk beater, if available. Sift cream of tartar into egg whites. Start beating whites at low speed, then gradually increase speed to high. Beat until whites are increased in volume and very foamy.
Gradually, add 1 cup sugar about 2 tablespoons at a time, reducing mixer speed to low while adding and sprinkling in each addition. Increase speed to high in between additions, and beat for about 15 seconds. When all sugar has been added, increase speed to high, and beat meringue just until stiff peaks form. It will be very thick. Remove from mixer. By hand, with large spatula, fold in ground nut mixture only until combined.
Now, working quickly, place small dabs of this meringue under the parchment paper at two opposite corners on each baking sheet, then replace parchment paper on sheet (the meringue will keep the parchment from sliding around). Place about half the meringue in the center of each traced circle. With a flat knife or large offset spatula, spread meringue out so that it just touches the traced circle on each piece of parchment. Make meringue surface as even as possible for each circle, but don't fuss with either for too long. As soon as one circle is done, place in preheated oven, then go back and form second circle, working quickly. Once second meringue circle is in oven, start timing. Bake the meringue disks for 2 hours, switching baking sheets back-to-front and rack-to-rack only after 90 minutes (do not open oven door before that time, or meringues may crack). TURN OVEN OFF, but allow meringues to dry out further in turned-off oven for at least 2 hours (overnight is is ok). At any time while meringues are baking or drying out, finely chop reserved 1/2 cup toasted, cooled nuts; set aside, covered.
When meringues have dried out for at least 2 hours, remove from oven. Very gently peel parchment paper from baking sheet; place parchment paper, with meringue still on it, on cooling rack. Cool completely. Now, place 10" diameter corrugated cardboard cake circle on top of one meringue. With large, very sharp, thin-bladed serrated knife, trim meringue circle, if necessary. To do so, work with a small section at a time, and saw overhanging edge of meringue very gently back-and-forth. Do not press knife down into meringue, and do not press down on cardboard cake circle. After trimming, loosen meringue from parchment with a long, thin, stiff-bladed spatula. If assembling dacquoise immediately, leave meringue circle on cooling rack. Otherwise, wrap airtight for storage. Repeat with other meringue disk.
To assemble dacquoise, have ready finely chopped nuts and a flat-bottomed serving plate or foil-lined corrugated cardboard circle at least 10" in diameter. For ganache: Combine finely chopped chocolates and salt in medium heatproof bowl. Place over hot water on low heat (water should not touch bottom of bowl); stir frequently until almost melted. Remove from heat and hot water; stir until smooth. All at once, add sour cream. With hand-held electric mixer at a low speed, beat in sour cream. If sour cream is at all cold, ganache will lump. If this happens, replace bowl over hot water and beat at a low speed almost constantly just until ganache is smooth. Remove from heat and hot water. Beat in vanilla. If ganache is too thin, allow to stand at room temperature, stirring occasionally, until of good spreading consistency. Otherwise, use immediately. Place a small dab of ganache in center of serving plate or foil-covered cardboard circle. Place the worse-looking of the two meringue circles, right side up, on the serving plate. By large spoonfuls, place about 3/4 of the ganache on top of the meringue circle; spread quickly to form an even layer. Place other meringue circle on top, right side up; press GENTLY to compact very slightly. Frost sides of dacquoise smoothly with remaining ganache (the top of this pastry is not frosted). Quickly, before ganache sets, coat sides with the finely chopped nuts, pressing them into the ganache lightly. Place in refrigerator. Chill at least 2 hours before serving.
To cut, use a large, sharp, straight-edged knife. Rinse the blade under hot water and dry it after every other cut or so. Allow the cut dacquoise to stand at room temperature for about 20 minutes, loosely covered, before serving. Just before serving, sift unsweetened cocoa powder (preferably Dutch process) very lightly over the top of each slice. Store any leftovers in refrigerator, tightly covered.