Heat the oven to 325 degrees. Place the cream, milk, vanilla seeds and pod, and cinnamon sticks, if using, in a medium saucepan, and bring to a boil. Remove from heat, cover with plastic wrap, and let sit for 30 minutes to infuse flavors.
Cut the brioche into 1/4-inch-thick slices. Cut slices into quarters, setting aside the rounded top pieces. Fill a 9-by-12-inch gratin dish or a deep oval roasting dish with the quartered pieces.
Return the milk mixture to a boil, remove from heat, and discard vanilla pod and cinnamon sticks. Add chocolate, and whisk until smooth. Combine egg yolks and sugar in a large bowl, and whisk to combine. Pour chocolate mixture very slowly into egg-yolk mixture, whisking constantly, until fully combined.
Slowly pour half the chocolate custard over bread, making sure all the bread is soaked. Arrange the reserved bread on top in a decorative pattern, and press firmly so bottom layer of bread absorbs chocolate mixture. Spoon remaining custard over bread until completely covered and all cracks are filled. Place a piece of plastic wrap over the dish; press down to soak bread thoroughly. Remove plastic, wipe edges of the dish with a damp towel, and allow to sit for 30 minutes. Place gratin dish in a larger pan; fill outer pan with hot water halfway up the sides of the gratin dish. Bake until set, about 35 minutes. Cool on a rack for 15 minutes.
Whisk the creme fraiche until soft peaks form. Serve pudding warm, garnished with creme fraiche and chocolate shavings.
A 9-by-13-inch glass baking dish may be used in place of a gratin dish.