Preheat oven to 170 C / Gas 3. Grease and flour a 23cm cake pan. Sift together the flour, baking powder, bicarbonate of soda and salt. Set aside.
In a large bowl, beat butter, caster sugar and dark brown sugar together until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the soured cream, mixing just until incorporated.
Pour mixture into prepared cake tin. Bake in the preheated oven for 75 minutes, or until a skewer inserted into the centre of the cake comes out clean. Remove cake from oven and immediately prick cake all over with a wooden cocktail stick 20 to 30 times. Pour butter sauce (see prep method below) over cake. Allow cake to cool completely as cake is very fragile when warm. Run a knife around the edges of the pan and turn out onto a serving dish. Dust with icing sugar.
To make the butter sauce: Heat 200g sugar, 110g butter, 4 tablespoons water and 1 tablespoon vanilla extract over medium heat until sugar dissolves, about 10 minutes. Do not boil.