For dough
3/4 cup warm milk (105–115°F)
1/2 cup plus 2 teaspoons sugar
3 teaspoons active dry yeast (from two 1/4-oz packages)
3 1/4 cups all-purpose flour plus additional for dusting
2 whole large eggs
1 large egg yolk
1 teaspoon pure vanilla extract
3/4 teaspoon salt
1 1/4 sticks (10 tablespoons) unsalted butter, cut into pieces and softened

For egg wash
1 large egg yolk
1 tablespoon heavy cream or whole milk

For chocolate filling
5 tablespoons unsalted butter, well softened
2 (3 1/2- to 4-oz) bars fine-quality bittersweet chocolate (no more than 60% cacao if marked), finely chopped
1/4 cup sugar
Special equipment: a stand mixer fitted with paddle attachment; 2 (8 3/4- by 4 1/2- by 2 3/4-inch) loaf pans; parchment paper