1. Heat oven to 350 degrees F. Butter bottom of 13 x 9-inch baking pan. In bowl of food processor fitted with metal blade, process almonds with 1 tablespoon of the sugar until finely ground; set aside. In medium bowl, stir together flour, cornstarch, and salt using wire whisk.
2. In large bowl, with electric mixer on medium-high, beat butter and remaining 2/3 cup sugar until light and creamy, about 3 minutes. Reduce mixer speed to low, and beat in yolks and almond extract. Gradually beat in flour mixture until blended.
3. Remove half the dough from mixing bowl, and knead it a few times until smooth; flatten into a 6-inch disk. Wrap disk in plastic wrap and refrigerate until firm, about 1 hour.
4. Beat ground almonds into remaining dough; press over bottom of prepared pan. Bake until golden brown, about 15 minutes. Cool 10 minutes. Spread dough evenly with jam.
5. Roll out flattened dough disk on lightly floured surface with floured rolling pin to 1/4-inch thickness. Using a fluted pastry wheel, cut out 1/4-inch-thick strips of dough. Crisscross strips over top of jam.
6. Bake 15 to 20 minutes, or until top crust is lightly browned around the edges. Remove pan to wire rack and cool completely. Sift powdered sugar over top, and cut into 32 squares.