1. Heat oven to 350 degrees F. Line two 8 x 8-inch baking pans with nonstick foil.
2. Melt 2 squares white baking chocolate in microwave on high for 1 minute. Stir until smooth. If needed, continue to melt in 15-second increments. Cool slightly.
3. Beat together butter and sugar, about 1 minute. Add cooled chocolate and vanilla; beat another minute. Add egg whites and milk, beating constantly. On low speed, beat in flour and salt. Fold in 1/4 cup of the crushed candy. Spread dough evenly between prepared pans (about 1-1/4 cups dough per pan), smoothing tops.
4. Bake at 350 degrees F for 13 to 15 minutes, or until top is dry. Use foil to lift bars from pans. Cool on wire racks.
5. Melt remaining 4 squares white chocolate. Spread half onto 1 cookie square. Top with second square. Spread with remaining chocolate; sprinkle with remaining 1/4 cup candy. Cut into 1-inch squares. Refrigerate until firm, about 1 hour. Makes 5 dozen squares.