In a medium saucepan, warm the milk over medium heat. Using a sharp knife, slice the vanilla bean vertically. Scrape out the seeds and add the pod and seeds to the milk. Cook until a skin forms. Remove the pan from the heat.
In a medium bowl, combine the sugar, flour, and salt, then whisk in the egg yolks.
Remove the pod from the milk mixture. In a thin stream, slowly whisk in the warm milk. Return this mixture back to the saucepan. Cook over medium heat until the mixture is extremely thick and gluey, about 5 minutes.
Press a piece of plastic wrap down over the surface of the cream to keep a skin from forming. Let cool and then refrigerate until ready to use.
In a small bowl, mix 3 tablespoons of flour with the yeast and then whisk in the water. Set it aside for 10 to 15 minutes or until it begins to get puffy.
Fit a large capacity food processor with the metal blade. Put in the remaining flour, yeast mixture, potato, egg yolks, vanilla and sour cream.
Process the ingredients for about 1 minute. Remove the dough from the working bowl and knead it by hand on a clean countertop for 1 minute. Return it to the food processor, running it for another minute and then kneading it on the counter for 30 seconds. Repeat this step until the dough is very smooth about 2 to 3 times.
Return the dough to the processor. Add the sugar and salt, kneading again in the processor and then on the counter until the sugar is dissolved. The dough will soften considerably and become very sticky.
Return the dough to the processor and add the butter process until well incorporated and then kneading it for another minute on the counter. The dough will cling to the sides of the processor and feel warm to touch at this point. Kneading on the counter should cool it and help firm it up.
Transfer the dough to a large bowl that has been lightly oiled. Cover and allow the dough to rise until approximately 3 to 4 times its volume, approximately 3 hours.
Line a baking sheet with parchment paper or butter it. Roll the dough on a floured surface into a rectangle measuring 13x16 inches and about 1/8 inch thick. Stir the chilled pastry cream and then spread it over the dough in a thin layer. Sprinkle the chocolate chips evenly over the dough. Roll long edge into a cylinder and pinch to seal.
Put the cylinder on the baking sheet. Using a sharp knife cut the cylinder down the middle lengthwise. Arrange the strips so the filling is facing up and they are lined up side by side. Working from the center, wrap them around each other to form a twist. Cover the dough and let rise until it's large and puffy, and remains indented when lightly pressed with your fingertip, about 45 minutes.
Preheat oven to 350 degrees F. Brush the dough with a beaten egg. Bake in the center of the oven until golden brown, approximately 35 minutes, turning the pan halfway through baking.
Cool on a rack for 1 hour before slicing.
Recipe originated in Fine Cooking Breads Winter 2015 edition.
This recipe was calculated using the exact brands and measurements I used to make this recipe. If you are following a strict diet please note changing anything will cause the nutritional info to change. My calculations are intended as a guide only.