Preparation

Make the pastry cream:

• 在中等锅中，用中火加热牛奶（如果您使用的香草豆，请立即添加），直到形成皮肤。取下锅。在一个中等混合碗中，将糖，面粉和盐混合。加入蛋黄，用木勺跳动。在温暖的牛奶中搅拌，不断搅拌。将牛奶混合物返回锅中。用中火煮，不断搅拌，直到混合物极浓密，粘稠（您需要切换到木勺），大约5分钟。如果您使用的是香草提取物，请立即搅拌。立即将糕点霜通过筛子迫​​使。轻轻将一张蜡纸或塑料按在热糕点奶油的表面上，以防止皮肤形成。 Let cool and then refrigerate until ready to use it.

• Line a heavy baking sheet (or an insulated sheet or two sheets sandwiched together) with parchment or butter it. Roll the dough into a rectangle about 13×16 inches and about 1/8 inch thick. Stir the chilled pastry cream to soften it and then spread it over the dough in a thin layer. Scatter the chocolate chips evenly over the surface. Roll the rectangle into a cylinder from the wider side and pinch the long edge to seal. Put the cylinder on the baking sheet. Cut the cylinder in half lengthwise, splitting it into two thin strips. Arrange the strips parallel to one another so that the filling is facing up, push them together, and wrap them around each other to form a twist, working from the center.
  

• Position a rack in the middle on the oven and heat the oven to 350°F. Cover the shaped dough and proof until it’s large, puffy, and remains indented when lightly pressed with your fingertip, about 45 minutes.
• Brush the braid with the beaten egg, taking care not to smear the filling or dislodge the chocolate bits. Bake until golden brown, about 35 minutes, rotating the pan halfway through baking. Let cool on a rack for 1 hour before slicing.

Make Ahead Tips

The dough can be made up to four days ahead of time and refrigerated, covered.