Line and grease 2 x 20cm springform tins. Preheat the oven to 180°C/gas mark 4. Put all the ingredients for the cake in a bowl then, using a handheld or stand mixer, mix everything together for 3 minutes until you have a smooth batter.
Divide the mixture between two tins and level off the tops. Bake for 1½ hours to 1¾ hours – check it after 1¼ hours as it will depend on your oven.
Meanwhile, make the buttercream. Add the butter to a mixing bowl and mix on high for 2 minutes. Add the icing sugar a little at a time and mix on a medium speed. Add the orange zest and milk and mix on a high speed for 3 minutes until light and fluffy.
When the cakes are golden brown, with a slight peak at the top, and a skewer inserted comes out clean, they are ready. Remove from the oven and leave to cool for 10 minutes in the tin. Turn out and leave to cool completely on a wire rack. Cut the domed tops off both sponges and level them off. Carefully slice both cakes across into two, giving you four sponges.
Place the orange juice and sugar in a jug and mix well. The sugar will not dissolve completely. Spoon the mixture equally over the 4 sponges.
Lay the four sponges out, making sure one of crusts faces down and one faces up. Generously cover the top and side of each with the buttercream. Put the crust-side-down slice on a cake base or plate, then carefully place the other layers on top, finishing with the crust-side-up slice. Place the whole cake in the fridge or in a cool place for 15-20 minutes until the buttercream has crusted over.
Add the grated fondant and 25ml water to a pan and melt over a low to medium heat, stirring continuously, until it has become a liquid. Add the food colouring a little at a time until you have a vibrant purple colour. Transfer to a bowl and leave to cool completely. Add a drop or two of cold water to loosen if needed.
Pour the cold purple fondant over the top of the cake, drizzling it gently down the sides. Finish off the decoration by brushing the tips of the sugar paste roses with edible glue and dipping them in edible glitter. Arrange them in a crescent shape around one edge of the top of the cake, or decorate it using real white roses.