1. Place butter and confectioners sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat until fluffy, 2 to 3 minutes. Add the egg, and beat until fluffy.

2. In a medium bowl, whisk together wheat germ, pastry flour, salt, and cornstarch. Add to butter mixture, and beat on low speed until combined, about 1 minute.

3. Divide dough into two equal parts; and place between 12-inch squares of parchment paper. Roll out each piece of dough into a 7-by-10-inch rectangle, about 1/4-inch thick.

4. Remove top pieces of parchment from dough; spread peanut butter over one rectangle. Invert second rectangle on top of first. Folding any overhanging edges of parchment; wrap dough. Chill at least 1 hour.

5. Heat oven to 350 degrees. Line two baking sheets with parchment. Unwrap dough. Generously sprinkle top and bottom with sugar. Trim edges, creating a clean rectangle. Cut into three dozen 3 1/2-by-1/2-inch rectangles; place on sheets, spaced 1 1/2 inches apart.

6. Bake cookies until barely golden, about 22 minutes. Transfer to a wire rack to cool. Bake or freeze remaining dough. Store in an airtight container up to 2 weeks.