Thaw the puff pastry (Pepperidge Farm brand) as directed on the package. It takes less than 40 minutes and there are microwave directions on the box if you're in a hurry.
Unfold the puff pastry on a cutting board and lay it in front of you with the 9.75 inch horizontal and the 10.5 inch side vertical. Cut vertical ½ inch strips. Aim for about 20. You'll need two strips for each horn. You can also do thicker strips and use 1 strip for each horn.
Have ready a parchment or nonstick foil lined tray or a couple of lined plates that will fit in your freezer. Also have ready a little beaten. For a little extra crunch, you can also sprinkle on sparkly sugar before baking, but that is optional.
Grease your molds very well with butter.
Take one strip of pastry and starting at the pointy end of the mold, coil/loop the pastry trip keeping sides touching. It will only go about half way, so take a second strip, pinching it to the first, and continue looping. The mold still might not be completely covered. Set the puff pastry wrapped mold on the tray and continue to make as many more horns as you cans. Set each one on a parchment lined tray or plate as you go. When done, put in the freezer for about 20 minutes.
Preheat the oven to 400 degrees F. When oven is hot, brush the frozen puff pastry with beaten egg and sprinkle lightly with sparkling sugar if using. Bake the puff pastry wrapped molds for about 18-20 minutes. Let cool, then carefully slide off the mold.
Once cool, take your white chocolate ganache out of the refrigerator. Using an electric mixer, beat it until thick enough to pipe. It might take a little longer than whipped cream does to thicken, but it will thicken. Just as it's starting to form peaks, taste test. Add powdered sugar to taste or leave it as is.
Fill a piping bag fitted with a round tip and squeeze the ganache into the cooled pastry. Sift powdered sugar over the pastries and serve. These so good when served fresh because the horn is puffy and crunchy and the cream is soft. However, you don't have to serve them immediately.
Store in the refrigerator for a few hours or eat immediately. For longer term storage, store the horns in a freezer bag in the freezer.