Ok, I have now made 2 mud cakes. One caramel, one vanilla. They are SO yummy!!!! 
I do have 2 problems.
The first problem is that the cake does not rise very well (it is not that big of a problem...I just wondered if it was normal)
the second problem, and this is a little bit of a problem, the edges get hard when it bakes. So after it is frosted and you go to cut into it you have to saw into it and it smooshes out all the icing in the middle. 
What, if anything, am I doing wrong???
TIA
~michelle~
I'm glad you asked this question. I made a chocolate one and I had the same problem with the edges being harder. I wonder if it's just a matter of turning our ovens down a bit. It must take the center longer to be done than the outer edges, due to the moisture content? Just a guess. Maybe Nati or Cakewiz or someone more familiar with these mud cakes can answer better than I. 
By the way, I love your avatar! 
Mud cake is a moist dense cake which is not a high riser.
If the outside edge of your mudcake is "hard" then the temperature is too high - it is cooked while the centre is not. Due to the density and moisture, it requires long slow cooking - not anything near 350oF as a temperature that is sometimes quoted.
Use 120oC [250oF] and cook it a lot longer - long slow cooking for a muddier cake. Usually add 15 mins to the cooking time for every 25oF the temperature is lowered.
Check this image for the "density"
http://sendachocolatecake.co.nz/xgraphics/website/bigslice.jpg
If the outside edge of your mudcake is "hard" then the temperature is too high - it is cooked while the centre is not. Due to the density and moisture, it requires long slow cooking - not anything near 350oF as a temperature that is sometimes quoted.
Use 120oC [250oF] and cook it a lot longer - long slow cooking for a muddier cake. Usually add 15 mins to the cooking time for every 25oF the temperature is lowered.
Check this image for the "density"
http://sendachocolatecake.co.nz/xgraphics/website/bigslice.jpg
Good picture... thanks!
Last week I too tried the White Chocolate Mud Cake... an extremely thick, thick batter that nearly had to be spread inside the pan. Is this normal??
Also, I lowered my oven temperature to the suggested 160C/320-325F -- and got the same results as were mentioned before... a very hard exterior while the center sadly sunk. Plus, these results were achieved in only 25-30 minutes. I shudder to think what would have happened had the cake baked for any longer. 
For now, I think I've pretty much got mudcake experimentations out of my system... unless (no offense to anyone) someone can come up with a more reliable recipe/technique/temperature.
Post your recipe.
I used the recipe that was posted in this thread... http://www.cakecentral.com/cake-decorating-ftopict-13269-mudcake.html
Two things first: Australian and American products are not going to be exactly the same, and check your ingredient amounts..
250gms margarine = 8 oz
150gms white chocolate buttons =5 oz
350gms dark brown sugar = 1 3/4 c
1 cup of milk (250mls)
2 large eggs
150gms self raising flour = 1 c
150gms plain flour = 1 c
Thre rearrangement of the flour proportions will not upset the chemistry as long as it totals 2 cups.
I have a problem with the brown sugar. My caramel mudcakes are almost exactly the same except the brown sugar amount is around 1-1 1/2 c not 1 3/4 c.
Also, molasses in the sugar provides more moisture in the cake chemistry - dark brown sugar has more molasses than ordinary brown sugar. What amount is in our brown sugar may not match what is in yours......
I would suggest cutting it back to 1 1/2.
Compare this one. It has less butter and less sugar: but made exactly the same:
CARAMEL MUD CAKE
185 g butter 6 oz
150 g white chocolate 5 oz
1 c brown sugar
1 c milk
1 1/2 c Flour
1/2 c SR Flour
2 eggs
Topping
250 g white chocolate
1/3 c cream
I would cook this 8" [3" deep] round cake pan 140oC/285oF for 2 hours.
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