Make the cake mix according to pack instructions, adding the blue and purple sprinkles to the raw batter.
Bake in a 7” round cake tin that has been greased and lined with baking parchment and leave to cool on a wire rack.
Double line the base of a 2-pint glass bowl with plastic wrap, leaving the edges quite long so that you can get the ice cream out easily.
Leave the purple ice cream to soften slightly and then spoon it into the bottom of the bowl. Smooth over and then put into the freezer for 2 hours to set firmly. Repeat with the blue layer-which should come to the top of the bowl approximately.
When the cake has cooled down, trim around the edge so that it fits the bowl exactly and press it down onto the ice cream. Put back in the freezer.
Using the fondant, make a face for the Yeti and set aside. Brush with water and sprinkle over the sanding sugar.
Stir the caster sugar and water in a small pan fitted with the sugar thermometer, until dissolved. Heat gently until the mixture starts to thicken and turn yellow (240F).
Whisk the egg whites and cream of tartar to soft peaks and slowly drizzle in the hot syrup, whisking constantly until the mixture is stiff and glossy. Spoon into a piping bag fitted with a large round nozzle.
Remove the ice cream from the freezer and upturn it onto a serving plate. Remove the plastic wrap and stick the face onto the side of the ice cream. Pipe the yeti’s fur all over in long spikes. Return to the freezer until ready to serve.
Just before serving, brown the meringue lightly with a Brule torch.