Line 2 cookie sheets or jelly roll pans with parchment. Place half the pasta bignè in a pastry bag fitted with a ½-inch plain tube (Ateco #6). Pipe 15 spheres onto one of the sheets or pans, each about ¾ inch in diameter, and space them about 1½ inches apart. Pipe the pasta bignè by holding the bag perpendicular to the pan or sheet and about 1 inch above it, squeezing out a sphere of the paste, then releasing the pressure and pulling away from the piped puff. If there are points of the paste in the center of the puffs, moisten a fingertip with water and press the points smooth. Repeat with the second half of the pasta bignè.
Bake the bignè at 375 degrees for about 20 minutes, until they are well risen and a deep golden color. Test one to see if it is sufficiently baked by removing it from the oven and cutting it open. If there are several filaments of the unbaked paste still clinging to the inside of the puff, continue baking for another 8 to 10 minutes. Remove the puffs to a rack to cool.
For the chocolate pastry cream, bring the milk and half the sugar to a boil in a 2-quart nonreactive saucepan over medium heat.
Whisk the egg yolks in a bowl with the salt and whisk in the remaining sugar in a stream. Sift the flour over the yolk mixture and whisk in.
When the milk boils, remove from the heat and whisk one-third of it into the yolk mixture. Return the remaining milk to a boil and, beginning to whisk the milk first, pour in the yolk mixture. Continue whisking constantly until the cream thickens and comes to a boil. Allow to boil, whisking constantly, for 30 seconds.
Remove from the heat and whisk in the chocolate, cinnamon, and orange zest. Let stand 2 minutes for the chocolate to melt completely, then whisk until smooth. Pour the cream into a clean nonreactive bowl. Press plastic wrap against the surface and refrigerate until cold. Immediately before filling the bignè, gently stir in the rum.
To fill the bignè, pierce each on the bottom with a small pastry tube or chopstick. Place half the filling in a pastry bag fitted with a ¼-inch plain tube and fill the bignè through the openings. Place them on a pan lined with clean parchment as they are filled. Repeat with the remaining filling and bignè. Cover loosely with plastic wrap and keep refrigerated until serving time. Immediately before serving, combine the cocoa and confectioners’ sugar and place in a fine strainer. Dust the bignè with the mixture.