She Creates Stuff has a great HOWTO that uses a spherical cake mold to make a ball-shaped, sealed pie-crust that you can stuff with your favorite jammy gooey goodness, and so celebrate the mysterious infinitude that is Pi.
To create it, I used a Wilton Sports Ball pan. This pan has two half spheres and two ring stands. It is intended to be used to bake a round cake but, instead, I formed a crust around it. Once baked, I filled the crust and cemented it together with white chocolate.Pi Day - Spherical Pie (Thanks, starshipminivan, via Submitterator!)
The trick to to create a barrier on one side of the filled crust by either securing a disc of cooked crust with the same chocolate that will be securing the two pieces together, or by making a chocolate disc and securing it to the crust on the inside. Allow the chocolate to solidify before continuing and assembling the sphere.