Canola oil spray (for loaf pan and parchment paper)

Dough:

¾ cup whole milk or half and half (105–115°F)

½ cup granulated white sugar, plus 2 teaspoons (divided)

3 teaspoons (2 packets, ¼ ounce each) active dry yeast

3 ¼ cups all-purpose flour, plus more for dusting

2 large eggs

1 large egg yolk

1 teaspoon pure vanilla extract

¾ teaspoon kosher salt

10 tablespoons (1 ¼ sticks) unsalted butter, room temperature or softened

Filling:

5 tablespoons unsalted butter, softened

4 ounces bittersweet or semisweet chocolate chips (or melting wafers)

¼ cup granulated white sugar

Egg Wash:

1 large egg yolk

1 tablespoon half and half (or heavy cream, or whole milk)

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Spray a 9×5-inch loaf pan with canola oil and line with parchment, leaving overhang. Lightly spray parchment.
Warm milk to 105–115°F. Stir in 2 tsp sugar, add yeast, and let sit 5–10 minutes until foamy.
In a bowl, combine flour, remaining ½ cup sugar, eggs, yolk, vanilla, and salt. Add yeast mixture and mix until dough comes together.
Add butter 1 tablespoon at a time, mixing well. Knead 8–10 minutes until smooth and elastic.
Place dough in greased bowl, cover, and rise until doubled, 1½–2 hours.
For filling: melt butter and chocolate until smooth, stir in sugar, and cool slightly until spreadable.
Punch down dough and roll into a 12×16-inch rectangle.
Spread filling evenly, leaving ½-inch border.
Roll tightly into a log. Slice lengthwise in half. Twist halves with cut sides up to form braided loaf.
Place in loaf pan, cover loosely, and rise 45–60 minutes until puffy.
Preheat oven to 350°F (175°C). Brush loaf with egg wash.
Bake 40–45 minutes, until golden brown and internal temp is 190°F.
Cool 15 minutes in pan, then transfer to wire rack to cool completely before slicing.

Dough can be refrigerated overnight for the first rise.

For a neat twist, chill the filled dough log slightly before slicing.

Bittersweet or semisweet chocolate works best for balance.

Brush with sugar syrup after baking for extra shine and moisture.

Nutella, nuts, or streusel topping can be used for variations.