In a stand mixer, combine flour, sugar, salt, and yeast. Add beaten eggs, knead, then incorporate butter until smooth.
Let the dough rise for 2 hours until doubled.
Heat milk, butter, and vanilla. Whisk egg yolks, sugar, cornstarch, and flour. Combine, thicken, and chill.
Roll out dough, spread custard, add chocolate chips, fold, cut into rectangles, and let rise.
Brush with egg mixture, bake at 350°F (180°C) for 10-12 minutes until golden. Brush with sugar syrup.
Let cool before serving.
