Of course you can burn a forest to make them, but what if you don’t have any trees? Can lava be used to burn down buildings?
Tips to Create a Burnt Building Husk
OK, the answer was Dehumidifier -> Solar Amplifier to start fires.
The encyclopedia page on the Solar Amplifier says you can only start fires in tundra and fynbos, but that’s not true. It can also be started in dry grass.
Not everything that’s logical works in this game, e.g. flooding with lava doesn’t burn down buildings nor set fire to dry grass.