How to Make It
Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, potato starch, and next 5 ingredients (through salt) in a medium bowl; stir with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add 1/4 cup milk; toss with a fork until moist. Press mixture into a 4-inch circle on 2 sheets of heavy-duty plastic wrap that overlap; cover with additional overlapping plastic wrap. Freeze 30 minutes.
Preheat oven to 350°.
Roll dough, still covered, into a 12-inch circle. Fit dough, plastic wrap side up, into a 9-inch pie plate coated with cooking spray. Remove remaining plastic wrap. Press dough against bottom and sides of pan. Flute decoratively. Pierce dough with a fork; bake at 350° for 18 minutes or until crust is golden brown. Cool completely on a wire rack.
Combine 2/3 cup sugar, 1/4 cup cornstarch, 1/4 teaspoon salt, and egg yolks in a medium saucepan; stir with a whisk until blended. Add 1 3/4 cups milk, stirring with a whisk. Bring to boil over medium-high heat, stirring occasionally with a whisk. Reduce heat to low, and cook 1 minute or until filling thickens.
Press filling through a fine sieve over a bowl; discard solids. Add chocolate, cocoa, 1 teaspoon butter, and vanilla to filling, stirring with a whisk. Cool 5 minutes.
Spoon filling into prepared crust. Cover surface of filling with plastic wrap coated with cooking spray. Chill 4 hours. Remove plastic wrap; spread whipped topping over filling.