It was a chestnut exploding in my microwave.
And now a recipe for chestnuts:
Get some chestnuts. If necessary get them out of those spiny cases, but if you've bought them from a shop someone will have done that for you already. Pierce the shiny (non-spiny) shells and then stick them in the microwave for a couple of minutes. Take them out when the first one explodes. Let the others cool a bit, and then peel them. Weird your friends out by telling them that you're eating mouse brains.
Then clean the microwave.