Preheat the oven to 150 C (300 F) on a fan-forced/convection oven. Or preheat to 160 C (325 F) on a standard oven.
Line the bottom and sides of a 20-cm (8-inch) loose-bottom cake tin with 10-cm (4-inch) sides.
Add the butter, chopped white chocolate and warm water to a large saucepan over low heat. Melt everything together while gently stirring.
Once smooth, remove from the heat and pour the mixture into a very large bowl. Set aside to cool.
In a separate large bowl, whisk together the sugar, flour, baking powder and salt.
Once the butter and chocolate mixture is no longer hot (it can still be warm), whisk in the eggs and vanilla extract. Then whisk in the sour cream until there are no lumps.
Working about ½ at a time, whisk the flour mixture into the liquid ingredients until smooth.
Pour the batter into the prepared pan.
Bake in the preheated oven for about 90 minutes for a 20-cm pan, or 75 minutes for a 23-cm tin. When the cake is done baking, an inserted toothpick should come out clean or with a few crumbs clinging to it. If you gently press down on the top of your cake with your finger, it shouldn't really give. If you're in doubt, it's better to bake for another 5-10 minutes - this cake is very unlikely to go dry.
Remove from the oven and cool fully before adding frosting or ganache
Invert the cooled cake onto a wire rack. If the top is rounded, optionally use a serrated knife to gently saw off the domed top. Only do this once the cake is 100% cooled.