Example ESP32 MQTT broker available via its own AP. No RPi-Mosquitto broker, cloud broker or wifi/router needed.
I don't claim to be an expert code monkey, but hope someone finds this useful. This is only possible due to the hard work of others so thanks to @terrorsl for https://github.com/terrorsl/sMQTTBroker and @knolleary for https://github.com/knolleary/pubsubclient