Preheat the oven to 180C. Grease and line the bottom of an 18cm round cake tin
Sift the flour and baking powder together and set aside.
Put the sugar and eggs in a large mixing bowl and beat until pale.
Add the buttermilk, vanilla extract and orange zest and continue to whisk. Then whisk in the melted butter.
Fold in the flour and then whisk briefly just to make sure it has combined.
Pour the mixture into the cake tin. Bake in the oven for 45 - 55 minutes.
Test whether it is cooked by inserting a skewer. It should come out dry. Let the cake cool for 5 minutes in the tin and then turn it out onto a wire rack to cool completely.
Make the icing by beating together all the ingredients except the sugar. Also add one tablespoon of the squeezed orange juice. As I used coarse shred marmalade I took out the pieces of peel to make a smooth icing. When smooth, add the sugar and continue to beat. Add a little extra icing sugar to make a thicker icing if it's too thin.
If you want, cut the cake in half horizontally and spread a third of the icing in the middle. Spread the rest of the icing onto the top of the cake.
Nutritional information is approximate and a guideline only.