Pre-heat the oven at 180 degrees C. Prepare a round cake form (23 cm diameter) by buttering the base and the sides. Line with baking paper.
Sift flour, baking powder and the pudding powder into a large mixing bowl. Add the eggs, sugar, butter, orange zest and juice to the dry ingredients and use a Braun hand mixer to whisk into a smooth thick mixture.
Pour the cake mixture into the cake form, even out with a rubber spatula and bake approx. 50 minutes. The cake is done when a toothpick or knife is inserted in the middle of the cake and it comes out clean.
Take out of the oven and allow to cool for 5 minutes in the form. This might be a difficult task and you will be dying to taste, but if you unmold it too soon the cake will break. Run a knife along the sides of the cake and tip out onto a cooling rack. Peel off the baking paper. Allow to cool completely before frosting the cake.
For the orange buttercream
In a small bowl mix all the ingredients together until thick and creamy using a Braun hand mixer. Spread the buttercream evenly over the top and sides of the cakes. Refrigerate for 30 minutes.
You can substitute the butter with 125g of cream cheese. This adds a wonderful delicate touch to the frosting.