Cream the softened butter with the caster sugar until light and fluffy, in a large bowl.
Add the eggs and beat well.
Fold in the flour and baking powder.
Add the pumpkin purée, spices and dried fruit and combine well.
Prepare your silicone baking pan by spraying with cake release spray or rubbing in a little butter.
Transfer the cake mixture into the silicon pan, and smooth the top.
Place the pan in your slow cooker.
Put an inch of water in the pot around the silicone pan.
Place a dry tea towel under the slow cooker lid to catch drips, put the lid on, and cook on high for 1.5 to 2 hours, until the cake centre is cooked through. Check using a knife or skewer or cake tester.
When done, lift the silicone pan out of the slow cooker and leave the cake to cool.
To make the icing, place the butter or spread in a bowl and whisk using an electric mixer. Once softened, add the pumpkin purée and whisk again.
Sift in half the icing sugar and mix again.
Add the remaining icing sugar and mix until you achieve the right consistency. If you find your icing is too runny, add a little more icing sugar.
Spread the icing on top of the cool cake, and serve.