Bake the Whole-Grain Wafers as directed; cool completely.
Lemon Cream Filling: In an electric mixer or food processor, blend the butter and sugar until smooth. Add the lemon zest and juice, mixing until smooth. Chill the filling, stirring every few minutes, until slightly firm but still spreadable, about 20 minutes.
Assembly: Top half the cooled cookies with the filling, using a small spatula to spread about 2 teaspoons of filling on the flat bottom of each cookie. Top each filled cookie with a plain cookie, pressing the flat bottom lightly onto the filling. Refrigerate briefly to firm up the filling; then store and serve at cool room temperature. (In warm weather, these cookies should be refrigerated.)