Butter and flour or line with parchment a round 23cm (10in) springform cake tin.
Place the diced apples and raisins in a large bowl and pour over the brown sugar. Let them sit for 30 minutes until they release some juice.
Beat the other ingredients thoroughly together until smooth in another bowl.
When the half an hour’s passed, add the cake mix to the apples and mix until combined.
Transfer the batter to the tin and smooth the surface with a spatula.
Bake for 50-55 minutes until a skewer inserted in the middle of the cake comes out clean.
Remove from the oven and cool in the tin.
To make the frosting, melt the sugar and butter in a small pan until dissolved. Add the salt, pour in the milk and the vanilla extract and bring to the boil. Take it off the heat and beat the icing sugar into the mix until smooth.
Spread on the cooled cake and let it set before removing the cake from the tin.
