Preheat oven to 375 degrees F. Combine flours and sugar in processor. Blend in butter using on/off turns until mixture is crumbly. Pat into bottom of 8x8x2-inch baking pan. Bake until shortbread just begins to color, 13 to 14 minutes. Remove from oven.
For caramel: Melt butter in heavy medium saucepan over low heat. Add sugar and corn syrup. Cook, swirling pan occasionally, until sugar melts. Increase heat to medium and cook until mixture registers 255 degrees F on candy thermometer (hard-ball stage), stirring constantly. Cool 30 seconds; stir in cream (be careful; mixture may bubble). Return pan to medium heat and stir until mixture registers 235 degrees F on candy thermometer (soft-ball stage). Pour hot caramel over shortbread. Cool to room temperature.
Spread melted chocolate evenly over caramel. Freeze until chocolate is firm, 10 to 15 minutes. Cut into 1-inch squares. Store in airtight container in cool place or refrigerate.
Makes about 5 dozen