1. To prepare crust, weigh or lightly spoon potato starch and flour into dry measuring cups; level with a knife. Combine potato starch, flour, 1 tablespoon sugar, and 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. Gently press mixture into a 4-inch circle on 2 sheets of heavy-duty plastic wrap that overlap; cover with additional overlapping plastic wrap. Chill 1 hour.
2. Preheat oven to 375°.
3. 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. Fold edges under or flute decoratively. Line bottom of dough with parchment paper; arrange pie weights or dried beans on paper. Bake at 375° for 22 minutes or until edges are lightly browned. Sprinkle chocolate chips over hot crust; spread with a spatula when melted. Cool completely.
4. To prepare filling, place egg yolks in a medium bowl. Combine 2 cups milk, 3/4 cup sugar, and cornstarch in a 4-quart saucepan. Bring to a boil; cook 1 minute, stirring constantly. Pour one-fourth of milk mixture into egg yolks, stirring with a whisk. Pour egg yolk mixture into milk mixture. Boil 2 to 3 minutes or until mixture thickens, stirring constantly. Remove from heat; stir in vanilla.
5. Pour filling mixture into cooled crust. Cover surface of filling with plastic wrap. Chill at least 3 hours. Sprinkle with grated chocolate.