Whisk together sugar, cornstarch, flour, ¼ cup water, and egg yolks in a medium saucepan. Whisk in milk; bring to a boil. Cook over low heat 2 minutes, whisking constantly; add vanilla. Pour into shallow dish; cover surface with plastic wrap. Cool slightly, then chill at least 2 hours.On parchment paper, roll puff pastry to 14 x 12 inch rectangle; pierce pastry with fork. Transfer parchment with pastry to a baking sheet. Cover with parchment and weigh down with another baking sheet. Bake at 400 for 15-18 minutes. Cool pastry and cut into thirds lengthwise.Blend powdered sugar with remaining water. Spread evenly over center pastry strip. Place melted chips in small resealable bag; snip off 1 corner. Pipe chocolate in crosswise stripes; pull knife through chocolate to form pattern. Let dry.