My poor wedding cake!

I have made this cake at this size before but I have no idea what's gone wrong! The crumb is very dark all the way through.

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The texture is fine, it's cooked through, it's not burnt, it tastes OK. It's just very dark and smells of biscuits!!
What could I have gotten wrong?

Things that have changed this time:

• Different oven
• I used I Can't Believe It's Not Butter spread instead of butter
• I put foil on top of the cake top stop it making a very high dome
• I guessed at the amount of baking soda
• Before the flour and after the eggs (creaming method) my batter looked like scrambled egg!!

Could the fat have burnt? Was it the baking soda? Did the foil trap steam?

The office have pretty much demolished it anyway.
Any advice greatly appreciated.

Thanks.