Red velvet cake

Name: Red velvet cake
Prep time: 1 h 5 min
Cook time: 1 h
Total time: 2 h 5 min
Cuisine: British
Ingredients: 300ml vegetable oil plus extra for the tins; 500g plain flour; 2 tbsp cocoa powder; 4 tsp baking powder; 2 tsp bicarbonate of soda; 560g light brown soft sugar; 1 tsp fine salt; 400ml buttermilk; 4 tsp vanilla extract; 30ml red food colouring gel or about ¼ tsp food colouring paste, (use a professional food colouring paste if you can, a natural liquid colouring won't work and may turn the sponge green); 4 large eggs; 250g slightly salted butter at room temperature; 750g icing sugar; 350g tub full-fat soft cheese; 1 tsp vanilla extract