Mud cake troubleshooting!

Hi
I’ve been trying to practise ganaching and covering cakes but unfortunately I can’t even seem to cook the mud cake! So frustrating!
I have used this recipe before:
http://www.bestrecipes.com.au/recipe/easiest-mud-cake-L2918.html?ref=recipe_collection_view%2Cchristmas-dessert-recipes

It worked perfectly in the past. But this time I tried making it in a high sided 6inch tin on 140C because we have a ‘conventional’ oven that heats a bit warmer than others.. Anyway it came out very crusty on top and then so moist that when I tried taking it out of the tin it started falling apart..

Tried making it a second time in a low sided 7inch tin and it came out burnt and very crumbly! Is there a good resource for trouble shooting mud cakes or how to work out cooking times for different cake tin sizes??