Just add a neutral fat, like canola oil.

I would add more softened fat like butter or maybe coconut oil, both of which are solid at room temp. This would help to thicken up the ganache without affecting the flavor.

The ratio of white chocolate to cream is way too low. I use a ratio of 3:1, white chocolate and cream. The butter looks fine. I'd suggest warming the existing ganache over a bain marie, and when warm, begin adding more white chocolate to bring it up to a ratio where it will set.

All of the suggestions above are going in the right direction, and remember the ganache will harden somewhat when it is cooled and off the heat.

Thanks, everyone. That was my first inclination. The first time I tried the ganache with Ghirardelli white chocolate chips, but I had no idea they contained no cocoa butter. The next time, I tried it with El Rey white cocoa discs. It was a little thicker the second time around, but still not thick enough to pipe as specified in the recipe. It chilled overnight in the fridge and was still not hard in the morning. Thanks for the suggestions, everyone!