The story broke late last year: you can use Vegemite as conductive paint. That means you can replace solder, Bare Conductive paint, or any other conductive substrate with it! I grew up with Marmite, which is the British version, so I'm perfectly happy "ruining" a pie with this for Pi Day.
The project we're going to adapt is this Touch Tone synth by Adafruit. I ended up modifying the sketch a bit, but it was really useful to start with their tutorial and code.
In SF, at least, it turns out you can buy Vegemite at most vaguely shmancy food shops. (No Marmite, though, sadly!) Pick up some toothpicks while you're at it. Listen to Land Down Under for cultural context.
The Circuit Playground Express (CPX) is ideal for this project because it has no components on the bottom = no lead in contact with your food!
To save the code, click the big pink Download button and save the .uf2 file to your computer. Hit the RESET button on the CPX while it's plugged into your computer, and it should flash red once, then turn green. This means you're ready to program, and it'll show up as the "CPLAYBOOT" flash drive on your computer. Drag your newly-created .uf2 file onto that drive, and in a few moments, you're ready to go! (Macs may complain about you supposedly removing the drive without ejecting it properly; this is, unfortunately, normal.)
If you want to actually play tunes on it, try my adapted code, which turns it into a 7-note C major scale. And if you want to REALLY go all-out, try modifying it so the A and B buttons change the octave!
You can even preview your code in the editor: the CPX image on the left is a fully-interactive simulation of the controller, so you can poke at it and see what happens before going through the whole download/flash process.
Use the toothpicks to attach the CPX to your empanada. It helps to insert them angled a bit toward the center, which keeps the board from moving up as well as sideways. Do this while the CPX is plugged in, so you won't need to disturb it again.
I found that my empanada was being recognized as a ground point somehow, so I used a toothpick to place big dollops of Vegemite next to the contacts I'd be using. Then, I coated another toothpick most of the way with the salty stuff, and used it as a stylus to connect the touch pads with their dollops. It worked!!
See above for the demo video, and check out the live-build thread here!