Preheat the oven to 180°C (160°C fan oven, 350°F, gas 4). Spray two, 3 x 20cm (8inch) diameter Victoria sandwich tins with Cake Release Spray to grease. Sift 75g (3oz) cocoa into a bowl and whisk in 350ml (12fl.oz) of the water.
Beat the margarine with 400g (14oz) sugar until well blended. Whisk in the eggs and the liquid cocoa mixture. Sift the flour and Baking Powder on top, and carefully blend the ingredients together to form a thick batter.
Divide the mixture between the tins and bake in the oven for about 20 minutes until risen and just firm to the touch. Turn out on to a wire rack to cool completely.
For the frosting, put the remaining sugar, Liquid Glucose and 115ml (4fl.oz) of water into a saucepan. Heat gently, stirring, until the sugar dissolves. Bring to the boil and boil until the temperature reaches 115°C (240°F) on a sugar thermometer. Remove from the heat.
In a large heatproof bowl, whisk the egg whites with the Cream of Tartar until very stiff – as if making meringue. Still whisking, carefully pour the sugar syrup on to the egg whites. Add the Natural Vanilla Extract and continue whisking for 4-5 minutes until glossy and stiff.
Use some of the frosting to sandwich the cakes together and put on a serving plate. Generously spread the remaining frosting all over the cake to cover. Dust with a little extra cocoa powder before serving.