Preheat oven to 160 C.
I used a silicon giant cup cake mould (I would use this for the top, as it is a lighter cake, if I cook it again.) Grease a pan if you are using metal and line with baking paper.
In a saucepan, put 600 grams good quality white chocolate, 400 grams of butter, 2 cups of milk (I used soy it was still good), and 1 1/2 cups of caster sugar, put it over a very low heat, and mix together until all the chocolate and butter have melted, and its become a shiny, silky smooth texture. It shouldn't take long. Set aside.
In a bowl beat 4 extra large eggs and 20 mls of Gluten Free vanilla. Making sure the chocolate mixture is cool, stir gently into the egg mix.
I find sifting gluten free flours pointless, as they as a rule are never lumpy. If you feel you must go ahead, use 200 grams of Self-Raising GF flour and 300 grams of plain flour. I always stir these together to make sure they are evenly distributed. Then add the white choc mix by half into the flour, mix through, then add the second half more slowly, folding it in. Pour into prepared cake pan. It will need at least 1 hour 20 minutes, but up to 1 hour 40 minutes. Using a metal skewer test if it runs clean you're set.
While cooling, melt another 400 grams of the white chocolate on a very low heat until completely melted, stir during whole phase. Stir in 200 grams of sour cream. When the cake is ready, run the ganache over the cake.
I adapted this recipe from www.exclusivelyfood.com.au into a larger recipe, and also made it gluten free, it worked a treat.