Line the base and sides of a 20 cm round cake tin with baking paper and lightly grease. Set aside until needed.
Add 1000 grams of water to your Thermomix bowl and place the chopped potatoes into the basket. Add the lid and cook for 14 minutes, varoma temperture, speed 2. Check the potatoes with a knife to see if they are cooked through.
Pre-heat your oven to 200 degrees.
Carefully remove the basket from your Thermomix bowl and drain the water.
Place the butterfly attachment in the bowl (carefully as the bowl will still be hot after cooking the potatoes) and return the potatoes to the bowl and add the caster sugar. Mix for 10 seconds, speed 4 to combine. If needed, mix for a further 5 seconds, speed 4 or until smooth.
Add the self raising flour, cinnamon, mixed spice, milk and coconut and mix for 10 seconds, speed 4.
Add the sultanas and raisins and select the kneading function and mix for 40 seconds.
Pour the mixture into your prepared cake tin and gently smooth the surface with a metal spoon/knife.
Place the Boston Bun into your pre-heated oven and bake for 35 - 40 minutes or until cooked through when tested with a skewer.
Remove the cooked bun from your oven and allow to cool for 15 minutes before carefully removing from the tin and placing onto a wire rack to allow to cool completely.
Once the bun has cooled, combine the icing ingredients in a small bowl and mix until smooth. You want your mixture to be stiff enough so that you can spread it onto the bun and it won't run down the sides.
Add the icing and sprinkle the coconut over the top.