In a small saucepan whisk together the 50g milk, 50g water, 20g bread flour.
Cook over medium heat stirring constantly until the mixture thickens into a smooth paste.
Remove from heat and let cool completely.
First, mix the warm milk, sugar, and yeast, let rest for 5 min, then add the eggs and vanilla bourbon paste (or vanilla extract).
Mix in the plain flour, rest of the bread flour, and salt.
Add the cooled Tangzhong and salt then knead for about 10 minutes until smooth.
Gradually add the soft butter kneading continuously until fully incorporated The dough may seem sticky at first but keep kneading it will become smooth and elastic.
Cover with a clean towel and let rest for 20 minutes.
Gently fold the chocolate chips into the dough until evenly distributed.
Shape the dough into a round loaf and place it on a parchment lined baking sheet.
Cover loosely and let the dough rise in a warm place for 1 to 2 hours or until doubled in size.
Preheat the oven to 350°F (180°C).
In a small bowl mix the egg yolk and milk then brush it over the dough. Sprinkle with caster sugar.
Bake for 25 minutes or until golden brown. If the top browns too quickly tent with aluminium foil halfway through baking.
Let cool slightly before slicing and enjoy warm.
