The temperature of the glaze has to be just right. Too hot and it won’t be thick enough; too cool and it won’t flow over the cake. Stir the ingredients for the glaze – don’t whisk as this will create air bubbles. You can make the sponge and the glaze mixture up to 3 days ahead but you’ll need to apply the glaze on the day you’ll be serving the cake. Store the baked sponge in an airtight container and the glaze in the fridge. To use glaze from the fridge, microwave for 10 seconds at a time until it’s just thin enough to pour.
15g powdered gelatine
200g granulated sugar
200g glucose syrup
150g sweetened condensed milk
200g good-quality white chocolate, chopped
3 different gel food colours
Soak the gelatine in 80ml cold water for 5 mins. Heat the sugar, glucose and 100ml water gently in a pan until the sugar dissolves. Bring to the boil until it reaches 103C. Remove from the heat. Add the soaked gelatine and stir until smooth. Stir in the condensed milk.
Put the white chocolate into a bowl. Pour over the warm condensed milk mixture and leave for 5 mins. Stir until smooth. Test the mixture with a sugar thermometer – when it cools to 30C it is ready to use. Stir occasionally as it cools to prevent a skin forming.
For a perfect finish, the glaze has to flow over the cake and drip off the bottom edge. For this, stand the cake on a plate smaller than its base, balance this on a small bowl, then put on a clingfilm-lined baking sheet.
Divide the glaze between three bowls to as many as you want. Stir gel food colour into each to make different shades. Pour over the cake, alternating colours so they form flowing patterns and cover the buttercream. Any excess will drip off the base. Leave to set for 25 mins. Transfer to a cake stand to serve.