Make cake. Pre-heat oven to 350 degrees. Invert the base of a 9” round springform pan so that it is lip side down (avoids having an indent in the base of the cake). Grease bottom and sides of pan and line bottom with parchment paper.
Melt chocolate in a small heavy pot over low heat, stirring occasionally. Set aside to cool.
Cream butter and sugars in a large bowl until like and fluffy. Add eggs one at a time, beating well between additions.
In a separate bowl, sift cocoa powder and add gluten-free flour, and salt and whisk until well combined. Beat into butter mixture. Stir in the melted chocolate and vanilla. Pour into springform pan and spread into an even layer.
Bake for 30 to 35 minutes or until set and slightly puffed in the centre. Let cool in the pan.
Make ganache. Place dark chocolate in a bowl. Place soy milk in a small pot and bring to boil over medium heat. Pour over chocolate and stir to melt.
Remove cake from pan and pour ganache over cake, ensuring the ganache spreads to the edge. Use a spatula to help spread evenly over the top and sides. Chill in the fridge to set.
Make white chocolate rectangles. Place white chocolate in a small, heavy pot over low heat. Melt slowly, stirring occasionally. When melted, use an off-set spatula to spread evenly on a parchment paper or Silpat sheet lined baking sheet. Place in fridge to harden.
Remove white chocolate from parchment paper and break into large pieces and press white chocolate against the sides of the cake, overlapping slightly. Stick leftover white chocolate pieces vertically into the middle of the cake to create a pattern. Chill again until ready to serve. Remove from fridge for 15 minutes before serving.