Warm ⅔ cup of whole milk to 110°F / 43°C. Whisk in 2¼ teaspoons of active dry yeast and 1 teaspoon of sugar, and leave for a few minutes, until foamy.
In a stand mixer, cream 5 tablespoons of softened butter with 3 tablespoons of sugar. Beat in 1½ tablespoons of neutral oil, 1½ teaspoons of vanilla extract, and 4 egg yolks, keeping one of the whites.
Add 2½ cups of all-purpose flour, 1 teaspoon of kosher salt, and the yeast mixture. Mix to a soft, slightly tacky dough, adding a little more flour if it is too sticky, and knead briefly.
Cover and let rise at room temperature for 1-2 hours, until roughly doubled. For the filling, mix 225g of finely chopped dark chocolate, 2 teaspoons of ground cinnamon, and ⅓ cup of cold butter by hand into a chunky paste.
Roll the dough into a 14 × 18in / 35 × 45cm rectangle, spread with the filling, and roll it up into a log. Stretch it to about 20in / 50cm, twist it into a figure 8, and set it in a lined 9 × 5in / 23 × 13cm loaf pan.
Cover and let rise for about 1 hour, until it fills the pan. Preheat the oven to 350°F / 175°C.
For the streusel, rub 2 tablespoons of cold cubed butter into ¼ cup of dark brown sugar, ¼ cup of all-purpose flour, and ¼ teaspoon of kosher salt.
Prick the loaf with a thin skewer. Brush it with the reserved egg white beaten with 1 tablespoon of whole milk, and sprinkle with the streusel.
Bake for 25 minutes, turn the pan around, and bake for 25-30 minutes more, until golden and the center reaches 185°F / 85°C. Cool before slicing.
