chocolate swirl buns

Name: chocolate swirl buns
Cuisine: European, German
Ingredients: 1/2 cup (120 ml) milk, preferably whole; 1/4 cup (50 grams) plus a pinch of granulated sugar; 1 1/2 teaspoons (5 grams) active dry yeast; 1 large egg, brought to room temperature; 2 cups (250 grams) all-purpose flour, plus more for work surface; 1/2 teaspoon table salt; 3 tablespoons (45 grams) unsalted butter, at room temperature, plus additional for bowl and muffin tins; 3 tablespoons (45 grams) unsalted butter, at room temperature; 1/4 cup (50 grams) granulated sugar; 1/2 pound (225 grams) semisweet chocolate; Pinch of salt; 3/4 teaspoon ground cinnamon (optional); Egg wash (optional); 1 egg; 2 teaspoons (10 ml) heavy cream or milk