Line the base and sides of a 26cm round springform cake tin (base measurement - spring form works best in this recipe) with baking paper
Preheat oven to 170 degrees fan forced and place rack into middle of oven
Place the butter into the container that you are going to mix the batter in (I use a large Tupperware mixing bowl as I can put it into the microwave ... see following point)
You want the butter to be just semi melted with some slightly liquid and the rest soft (looking like a lovely thick custard), and the easiest way for me to do this is to put the container with the butter into the microwave for 20 seconds to soften the butter down a little ... if it is very cold and the butter hasn't softened enough, then continue for another 5 seconds until you get the desired finish (see photo)
In another container add the yoghurt, milk, vanilla and eggs
In a large bowl sift the flour, sugar and custard powder (this is important to do)
Now you are ready to mix everything together
Firstly, use your electric mixer to mix the yoghurt/milk/vanilla/eggs for a few seconds
Then place the beaters into the softened butter and start mixing on medium speed until lovely and creamy
Slowly pour the dairy mixture into the butter and continue until it is all mixed in well
Once that is done remove the mixer for a minute and put aside
Add all of the flour mix to the liquid ... it will look like it is too much but all is okay
Place beaters into the mix and start beating, moving the beaters up and down and slowly incorporating the liquid from the bottom
Keep moving the beaters around beating for at least 2 minutes ... during this time you will see the batter start to come together and fluff up
Do not beat for any less than 2 minutes, but it may be that you will beat it for up to 3 minutes depending on the fluidity of your butter
Pour the batter into the cake tin and place your chosen fruit on top (I like to place my fruit strategically so that when the cake is turned out it looks nice and even, but you can do whatever pleases you)
Place cake into oven and bake for 1 hour and 10 minutes, check the centre of the cake and if necessary bake for a further 10 minutes (sometimes it takes less time, and sometimes more time, it all depends on the fruit being used but mostly I bake it for 1 hour 20 minutes)
Remove cake from oven and allow to rest for 15 minutes
Have your serving plate ready, place it over the cake and invert the tin so that the cake drops out onto it
Unclip the sides (but do not remove yet) and remove the base
Whilst the sides are still in place you can now move the cake around on your platter to ensure it is placed evenly on the plate
Remove the sides and dust with cinnamon sugar
Allow the cake to rest for another 30 minutes or so before dusting with cinnamon sugar again before serving
Keeps well in a covered container at room temperature for three to four days