Preheat the oven to 170°C (150°C Fan, Gas Mark 3). Lightly grease 3 x 18cm (7inch) shallow cake tins.
Whisk the egg yolks with the caster sugar until pale, thick and creamy. In another bowl, whisk the egg whites to form soft peaks.
Sift the potato flour and Baking Powder on top of the whisked egg yolks and carefully mix together.
Add the rose flavour and egg whites, then gently fold into the mixture until well blended.
Spoon one third of the mixture into one of the prepared tins and smooth the top. Divide the remaining mixture equally between 2 bowls. Colour one with Pink Gel Food Colour (about 25 drops) and the other with Violet Gel Food Colour (about 55 drops).
Spoon the coloured mixtures into the other 2 prepared tins, smooth the tops and bake all the sponges for 20-25 minutes until risen, lightly golden and just firm to the touch – the sponge will shrink slightly from the edge of the tin when cooked. Leave to cool for 5 minutes then turn on to a wire rack to cool completely.
To decorate, make up the Egg White Powder according to the packet instructions in a heatproof bowl, then whisk until stiff and dry. Set aside.
Put the sugar in a saucepan with the Liquid Glucose, 60ml (4 tbsp) water and the salt. Heat gently, stirring until dissolved, then raise the heat and cook until the mixture reaches a temperature of 115°C (240°F) on a sugar thermometer.
Whilst whisking the egg whites, carefully pour the hot sugar syrup on top. Continue whisking for 1-2 minutes until the mixture is thick and glossy like meringue. Mix in the Rose Flavour.
Place the violet coloured sponge on a serving plate and generously spread with the meringue frosting. Top with the pink sponge and spread with more frosting. Sit the plain sponge on top and press down gently to make sure all the layers are secure.
Using a spatula or palette knife, generously spread the remaining frosting over the top and sides of the sponge. Sprinkle with Violet Crystals and decorate with edible flowers. Your cake is now ready to serve and enjoy!