Florence is my mother-in-law Darina’s aunt, and this cake is legendary at Ballymaloe. It keeps very well for a couple of weeks.
Preheat the oven to 180ºC (350ºF), Gas mark 4. Brush the sides of the baking tins (Two 20cm (8in) diameter sandwich tins or 28cm (11in) diameter springform/ loose-bottomed cake tin) with melted butter and dust with flour. Line the base of each with greaseproof or parchment paper.
Cream the 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 one at a time, beating well between each addition. Sift the flour and baking powder into the mixture and stir in gently, then stir in 1 tablespoon of the orange juice.
Divide the mixture evenly between the two tins (or pour into the single tin), making a slight hollow in the centre of each cake so that it will rise evenly without forming a peak. Bake in the oven for 25–30 minutes or until a skewer comes out clean when inserted into the centre of the cakes. Allow to sit for 5 minutes then turn out onto a wire rack and allow to cool.
Meanwhile, make the orange filling. Cream the butter until very soft, then add the icing sugar and orange zest. Gradually beat in 1 tablespoon of the orange juice to create a smooth, soft mixture.
To make the glacé icing, add enough orange juice to the icing 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 in half horizontally (or the single cake into two or three, depending on its thickness), spread with a little of the orange filling and sandwich the pieces together. Spread icing over the top and sides and decorate with the crystallised flowers (if using).