Carefully remove all paper from your panettone. Slice the panettone into even 1/2-inch thick pieces.
Line a freezer-safe bowl (about 8-9 inches wide) with plastic wrap, leaving plenty of overhang to cover the cake later. Place one panettone slice at the bottom of the bowl, gently pressing it down.
Arrange more panettone slices around the sides of the bowl, overlapping them slightly to ensure no gaps. Press them gently against the bowl to form a solid shell.
In a stand mixer, combine the heavy cream, sweetened condensed milk, sugar, vanilla extract, and Cointreau (if using). Beat on medium-high speed until the mixture forms soft peaks. Be careful not to overmix.
Pour the creamy semifreddo mixture into the panettone-lined bowl. Gently fold any overhanging panettone slices inwards over the semifreddo; it doesn't need to be perfectly neat.
Place one or more panettone slices on top of the semifreddo to fully cover it.
Fold the plastic wrap overhang completely over the panettone bombe to seal it. Freeze for at least 8 hours, or ideally overnight, until firm. This is a great make-ahead dessert!
When ready to serve, melt the dark chocolate. You can do this in a heatproof bowl set over a pot of simmering water (double boiler method) or carefully in the microwave.
Remove the panettone bombe from the bowl and unwrap the plastic. Place it on a wire rack set over a baking sheet. Pour the melted chocolate evenly over the top of the bombe, letting it drip down the sides. Immediately decorate with festive sprinkles.
The chocolate shell will harden quickly, usually within a minute or two. Your beautiful Panettone Bombe is now ready to slice and enjoy! Let it sit out for 5-10 minutes for easier slicing.