Preheat the oven to gas mark 4/350oF/180oC/160oC fan and line two 6 inch baking tins with baking paper. Into a bowl, sift in the self-raising flour, bicarbonate soda and salt then set aside.
In another bowl or stand mixer bowl, cream together the butter and sugar until fluffy. This will take around 4 minutes. Then add the vegetable oil and fully combine. Mix in the eggs then mix in the vanilla extract, sour cream and milk. Once everything is well combined, add in the dry ingredient set aside earlier and mix until you have a smooth batter.
Divide the batter between the two cake tins and bake for 25 – 30 minutes. Allow the cakes to cool in the tins for 10 -15 minutes then remove and place the cakes on a cooling rack to completely cool.
Using the paddle on a stand mixer, beat the butter for around 3 minutes until fluffy then gradually add in the icing sugar and mix until fully combined. Once all the icing sugar is fully combined, add in the double cream, vanilla extract and food colouring if using. Continue to mix until the buttercream is smooth and creamy.
Place one of the cakes onto a turntable or plate then add some buttercream on top, spreading it evenly onto the cake. The cake is very delicate so be gentle during this process. Place the second cake on top of the buttercream and then frost the whole cake using a small palette knife. You may need to do a crumb coat first (see recipe tips). Once the cake is frosted, decorate with sprinkles and enjoy.