In a stand mixer with dough hook, combine yeast mixture, tangzhong, melted butter, vanilla, eggs, salt, cocoa powder, and both flours. Mix until a soft dough forms.
1/4 cup unsalted butter, 2 tsp vanilla bean paste, 2 large eggs, 1 1/4 tsp salt, 1/4 cup Dutch-process cocoa powder, 2 1/4 cups all-purpose flour, 1 cup bread flour