Place the flour, baking powder, baking soda and the salt together in a large bowl and mix this.
In another bowl add the yogurt, sugar, egg and the orange juice. Stir this through and add the flour mixture and mix it well. Add the chopped rhubarb, the chocolate (if using) hazelnuts and the orange zest. Stir this through with a spoon or spatula
Put into a greased cake tin and bake in the preheated oven for about 50 minutes. Check with a wooden skewer if the cake is done. When it comes out clean it will be cooked. If not leave to stand in the oven for a few more minutes.
For making the icing put the icing sugar into a bowl and add a little bit of water to it. Don't use too much water all in one go as it dissolved and if it becomes too runny it will be very messy to apply.
Once at the right consistency brush it over the top of the cake and sprinkle with the toasted almonds