Pre-heat the oven to 175ºC (350ºF). Grease and line the bottoms of two 8 inch cake tins. Melt the white chocolate and set to one side.
Beat the butter and sugar together until light and fluffy. Add the eggs one at a time, beating between each egg. Add the melted chocolate and sour cream (gräddfil), and mix to evenly combine.
Sift together the flour, baking soda, salt (if using), baking powder and vanilla sugar. Gradually mix into the wet ingredients a bit at a time.
Divide evenly between the two cake tins and bake for around 25-30 minutes until the tops bounce back when gently pushed and an inserted skewer comes out clean. Leave to cool for a while in their tins then turn out onto a wire rack to cool completely.