I'm going to let you know how I decorated this. I will assume you know how to make cake and ganache and fondant, if you don't you can buy these things ready made or can most likely get an instruction on here.
I will go through the harder things.
Step 1: Cake
To make the sponge I used a five egg mixture for each half of the sphere. This didn't reach the top of the tin, hence the use of a round tin to fill the gap (this only needed a 3 egg mix).
I joined the three sections once cooled with plain chocolate ganache as this sets better for holding shape. I covered the completed ball in white chocolate ganache which was coloured orange. I rolled out some fondant and cut out my pumpkin face and placed it on. I then sat it in the fridge to set.