MAKE THE STARTER: About 6 to 10 hours before mixing your dough, combine the starter, water, and flour in a clean container. Cover and leave at room temperature until doubled in volume and bubbly.
WARM THE MILK: Heat the milk until it reaches a temperature of 110°F to 120°F. Let cool to 80-100°F before continuing with next steps.
WEIGH THE INGREDIENTS: Tare a mixing bowl, then combine 120 grams of warm milk, 100 grams of starter, and the sugar. Allow the sugar to dissolve. Then, add the flour and the orange zest.
MIX: Mix the ingredients together until a shaggy dough is formed, then add the melted butter and the salt. Mix to combine.
KNEAD: Turn the dough out onto a work surface and knead for 10 to 15 minutes by hand or 4 to 8 minutes with a dough hook in a stand mixer on the lowest two speeds, until the dough is smooth, is no longer sticky, and releases easily from the bowl or work surface.
BULK FERMENT: Cover the dough and ferment for 6 to 10 hours, until doubled in volume.
PREPARE THE BAKING SHEETS: Line two flat baking sheets with parchment paper and set aside.
PREPARE THE FILLING: Place the chocolate chips in a glass or other microwave-safe bowl and microwave for 30 seconds. Add the butter and sugar and stir until smooth. Set aside.
DIVIDE: On an unfloured work surface, divide the dough into 8 pieces (about 119 grams each).
SHAPE: With a rolling pin, roll a piece of dough into a 5-by-9-inch rectangle. Brush on about ½ tablespoon (15 grams) of filling, leaving a ½-inch border along the edges. Roll the dough up from one of the long sides to the other to make a 9-inch-long rope. Use a serrated bread knife to cut the rope of dough down the center from one end to the other, leaving a ½-inch section at the end uncut. Twist the cut ends around one another three times with the inside layers facing out. Wrap the end of the twisted end around the intact end and pinch under to seal together in a twisted babka shape. Repeat with the remaining dough. Place the babka on the baking sheet 2 inches apart.
PROOF: Cover and proof for 2 ½ to 5 hours, until about 1½ times larger in volume. Touching the dough should leave an indentation.
PREHEAT: Preheat the oven to 375°F.
BAKE: Place the first pan in the oven, reduce the heat to 350°F, and bake for 20 to 24 minutes, until the bread is golden brown. Move the finished babkas to a cooling rack. Repeat this step with the second pan.
COOL: Cool for 15 to 20 minutes on a cooling rack before serving.