AThis is an australian mudcake What would make the cake form that square shape on the top and then fill a smaller square area with bubbles and different texture? I tested and its completely cooked [IMG]http://www.cakecentral.com/content/type/61/id/3295091/width/200/height/400[/IMG]

It's fine - eat up!

I don't know why - maybe the inside takes longer to cook? But mine often look like that and they're just fine.

...and that top surface often goes crusty as well and tastes extra delicious.

I think for that size of pan (not sure what size it is exactly, but it look's over 8") you should probably use a heating core to ensure even stable baking throughout. I'm sure it will be fine, but maybe for the furture throw a heating core in there to even things out. 

AThat's precisely the crust pattern my mud cakes get too. I do think the outer area which is smoother is setting up faster, and the inner section (which takes longer to cook) has more time to develop bubbles which then set in to the upper surface of the cake.

A mud cake is really heavy on sugar (and chocolate obviously!) so it seems to develop this caramelised upper crust. Some people get rid of it, it depends on how much my cake domes and how crusty it is - it's really good for snacking if you do cut it off. Baker's perks!

ADo yours also generally sink in the middle???

Ill cut off the crust it's just a shame to always be cutting down cakes even further, I have to bake double just to get a decent height!

Mine sometimes sink, yes. I've found covering my mudcakes with foil through the whole cooking process makes them rise evenly, and keeps the top soft.