Make Tangzhong: Whisk together 2 tablespoons of bread flour and 1/3 cup of water in a saucepan over medium heat for 3-4 minutes until thickened, then cool.

Prepare Dough: Combine 3 ½ cups bread flour, ¼ cup sugar, 1 teaspoon salt, and 2 ¼ teaspoons yeast in a stand mixer. Add 1 cup milk, 1 large egg, and cooled tangzhong. Mix for 5 minutes, then add ½ cup softened butter and knead for 15 minutes.

Ferment Overnight: Transfer dough to an oiled container, cover with plastic wrap, and refrigerate for 8 to 12 hours.

Make Filling: Melt ½ cup dark chocolate and 2 tablespoons unsalted butter in a microwave. Stir in ½ cup powdered sugar, 2 tablespoons cocoa powder, and a pinch of flaky sea salt until glossy.

Shape Babka: Roll out the cold dough into a 12 by 15-inch rectangle. Spread the filling, sprinkle with ½ cup chopped pistachios, and roll tightly into a log.

Braid Dough: Twist the dough halves together, pinch ends to seal, shape into a coil, and place in a greased loaf pan.

Final Proof: Cover the dough with a towel and let proof in a warm area for 60 to 90 minutes.

Bake: Preheat oven to 350°F (180°C) and bake for 30-35 minutes until golden brown and internal temperature reaches 190°F (88°C).

Sugar Syrup Finish: Simmer ¼ cup sugar and ¼ cup water until dissolved. Brush syrup over the babka after baking.

Cool and Serve: Let the babka cool in the pan for 20 minutes before transferring to a wire rack. Slice and enjoy.