For a rich, chocolaty babka loaf, we started with a yeasted dough that we flavored with orange zest and vanilla. Then we made a luscious chocolate filling by microwaving butter and chocolate together and stirring in confectioners’ sugar, cocoa, and salt until the filling was smooth. To combine them, we rolled the dough out to an 18 by 12-inch rectangle, spread the filling over it, and rolled it in