Preheat the oven to 110°C (90°C Fan, Gas Mark ¼). Line 2 large baking trays with baking parchment. Make up the Egg White Powder as directed on pack, or use egg whites and transfer to a large grease-free bowl.
Whisk until very stiff and dry. Whisk in the caster sugar in 4 batches, then sift the icing sugar on top and add the Vanilla Extract. Using a large metal spoon, gently fold the icing sugar into the mixture to make a thick, glossy meringue.
If you want to make more than one colour of meringue, spoon some of the mixture into separate bowls. Squeeze a few drops of Gel Food Colour over the top of the meringue.
Using a large spoon, gently turn the spoon over in the meringue and lift out a large scoop. Place on the baking tray in an oval shaped mound about 5cm (2inches) high. You should see the colour marbled through the meringue. Add more Gel Food Colour as necessary; you should be able to make 8 meringues.
Bake in the oven for about 2 ½ hours until the meringues are crisp on the outside, but still perfectly white – prop the oven door open slightly if they start to discolour. Transfer to a wire rack to cool. The meringues will be crisp on the outside, but still retain a little softness in the middle.