Preheat and oven to 350 degrees F. Set aside a parchment-lined baking sheet. Draw two 8-inch circles on the parchment, leaving at least 1 inch between them. (You may need to use two baking sheets.).
TO MAKE THE MERINGUES: In the clean bowl of a standing mixer fitted with a whisk, on medium low speed, beat the egg whites until frothy, about 1 1/2 minutes. Increase the speed to medium-high, add the salt and the vinegar, and slowly add the sugar, whisking until thick, about 2 1/2 minutes. Increase the speed to high and whisk until stiff and glossy, about 4 minutes. Sprinkle the cornstarch over the whites during the last minute of beating. Divide the meringue between the parchment circles and gently spread out evenly within each circle.
Reduce heat to 300 degrees F. Bake in the middle of the oven for 1 hour. Turn off the oven and leave the meringues inside, with the door shut, until completely cool, about 4 hours. The meringue will be almond in color.
TO MAKE THE LEMON CURD MOUSSE: In a bowl with an electric mixer, on medium speed, beat the egg yolks and sugar until pale yellow, about 2 minutes. Add the juice, zest, and butter. Transfer to a saucepan. The mixture may appear curdy. Cook over low heat, stirring with a wooden spoon until the mixture becomes smooth and thickens, about 10 minutes. Do not overcook or it will separate. Remove from heat. Pour into a bowl and allow to cool. Cover and refrigerate until you are ready to assemble. (Makes about 1 1/2 cups.).
When you are ready to assemble the cake, whisk the cream in a bowl with an electric mixer until stiff peaks form. Fold in the lemon curd.
TO ASSEMBLE: Place a dollop of the mousse on the cake platter. (This will help keep the meringue stable.) Place one of the meringues on the platter, pressing lightly. Spread half the mousse on top of the bottom layer. Add the second meringue and top it with the remaining mousse. Garnish the cake and platter with raspberries and blossoms.