Place the chocolate, butter and corn syrup in a medium heatproof bowl. Double boil until almost melted. Remove the glaze from heat and set aside to finish melting. Stirring once or twice until perfectly smooth.
Cool glaze, without stirring until nearly set and consistency of easily spreadable.
To glaze and marble the torte:
1. Transfer up to a quarter of cooled glaze to another bowl. Use a metal icing spatula to spread the transferred glaze over the torte. Crumb coat a thin layer. Refrigerate the crumb-coated cake about 10mins (do not leave torte in the fridge for any longer, or else torte will be too cold and glaze will not be pretty)
2. Melt the white and milk chocolate in two separate heatproof cups on double boiler. Stir until smooth. Remove from heat and set aside until needed.
3. Re-warm the remaining of the cooled glaze until 32-33C. Pour through a fine strainer.
4. Centre the torte on a turntable. Pour all the glaze on the top of the centre. Working quickly and turn the turntable. Use just 2-3 spatula strokes to spread the glaze over the top of the torte so that it runs down over the sides.
5. While glaze still fluid, quickly drizzle the white and milk chocolate over the top. Use a drag artist’s brush (small brush) through the glaze to create marble pattern.
6. Leave the glaze to set at room temperature (10-20 minutes).