Buildapc

Easiest way is to enable Restore on AC/Power Loss if your bios has that feature and use a wifi plug to do a power cycle whenever you want to start it. I believe this would also works even if the computer is powered off.

I haven't fully attempted this yet however this appears to be achievable on Android via Tasker using the Voice Command Capture project in combination with a remote control app - I'm using Unified Remote which features a Wake on Lan command which I've setup and know to work.

Hope this helps!