Preheat the oven to 350 degrees F. Brush the sides of 2 (8-inch) sandwich or cake pans or a (11-inch) springform pan with melted butter, and dust with flour. Line the base of each pan with greaseproof or parchment paper.
Cream the 2 sticks butter in a large bowl or in an electric food mixer until soft. Gradually add the sugar, and orange zest, and continue to beat until the mixture is light and fluffy. Add the eggs, 1 at a time, beating well between each addition. Sift the 8 ounces flour and baking powder into the mixture, and stir. Gently, stir in 1 tablespoon of the orange juice.
Divide the mixture evenly between the 2 prepared pans, making a slight hollow in the center of each cake so that it will rise evenly without forming a peak. Bake in the oven until a skewer comes out clean when inserted into the center of the cakes, about 25 to 30 minutes. Let the cakes sit for 5 minutes, and then turn out onto a wire rack, and cool completely.
To make the orange filling: Cream the butter until very soft, and then add the sugar and orange zest. Gradually beat in 1 tablespoon of the orange juice to create a smooth, soft mixture.
To make the glace icing: Add enough orange juice to the sugar for a spreadable icing. If it is too soft, it will run off the cake, but if it is too stiff, it will be difficult to spread.
When the cakes are cool, carefully slice each cake in half horizontally. If baking a single cake, slice the cake horizontally into two or three even layers. Spread the orange filling between the cake layers, and sandwich the pieces together. Using a palette knife, spread the glace icing over the top and sides, and decorate with the crystallized flowers, if using.
Recipe courtesy of Rachel Allen