Spoon over the remaining half of the cake mix, level the surface and scatter the remaining rhubarb and raspberries over the top (do not press in).
Bake for 45-50 minutes or until it’s not wobbly in the centre, barely springs back to touch and a skewer inserted comes out with a few crumbs on it.
Beat the butter until pale and creamy, about 5 minutes. Add the icing sugar and beat to fully combine. Gradually add the cream cheese, a small cube at a time, until fully combined. Add the vanilla and orange zest and mix to just combine.
Spoon the frosting into the middle of the cake and use an offset spatula or the back of the spoon to spread in a circular pattern, gradually spreading towards the edges. Decorate with roast rhubarb (optional) or raspberries.
Preheat the oven to 180°C.
Lay the rhubarb in a single layer in a medium baking pan lined with baking paper - it just needs to be large enough so the rhubarb can all lie flat.
In a small bowl, combine the brown sugar, orange juice, vanilla and water. Pour over the rhubarb.
Top the rhubarb with another sheet of baking paper (this helps it soften without browning) and bake for 15-20 minutes or until tender.
Remove from the oven and set aside to cool. Depending on how you are serving this, you can also cook the remaining liquid down separately to make a syrup.