FOR THE SPONGE

1. Preheat your oven to 180°C/ 160°C fan oven/ Gas Mark 4. Grease and line 4x 7 inch cake tins. In a large bowl whisk together the melted butter and oil until blended. Add the water then whisk again until well blended.
2. In a jug beat together the eggs, vanilla and soured cream. Add to the butter mix and whisk again.
3. In a separate bowl mix together the flour, cocoa powder, caster sugar, light brown sugar and the bicarbonate of soda and baking powder. Add to the wet ingredients and whisk until well mixed.
4. Divide the cake batter between the cake tins and bake in the oven for 25-30 minutes until cooked. To test if the cake is cooked insert a skewer and if it comes out clean then the cakes are ready. Take the cakes out of the oven and cool in their tins for 30 minutes then remove from the tins and cool completely.

FOR THE WHITE CHOCOLATE BUTTERCREAM FILLING

1. To make the white chocolate and vanilla bean buttercream filling, melt the white chocolate in a glass bowl in the microwave in 20 second bursts, stirring in between until melted. Leave to cool for 10-15 minutes until room temperature but still fluid. Put the butter into a large bowl and beat with an electric whisk or freestanding mixer until light and fluffy.
2. Sift in the icing sugar then add the vanilla bean paste and beat again until well incorporated. Add the melted white chocolate and beat together for a couple of minutes until light and fluffy in texture.

FOR THE VANILLA SWISS MERINGUE BUTTERCREAM

1. To make the vanilla swiss meringue buttercream coating, fill a pan with water so it is 1/3 full and bring to the boil. In a large heatproof bowl or the bowl of a freestanding mixer combine the egg whites, sugar and salt. Put the bowl over the pan of water (this is called a bain marie) making sure the bottom of the bowl does not touch the water and whisk the egg whites and sugar together for a minute or two until the mixture is frothy, the sugar has dissolved and it is warm to the touch.
2. Remove from the pan and whisk with the freestanding mixer or a hand held electric whisk until the mixture forms medium soft peaks and is cooled to room temperature. With the mixer running on medium high gradually add the softened butter a table spoon at a time until it is all incorporated and beat for a few minutes until light in colour. The mixture may look like it is curdling but keep whisking and it will eventually combine to make a smooth buttercream.