Preheat your oven to 180C/160C fan/Gas Mark 4. Grease 2 x 20cm cake tins.
Mix togther the cake mix, vegetable oil, water and eggs for 2-3 minutes until smooth and creamy. This can be done by hand or with an electric mixer.
Divide the mixture evenly between the two cake tins. Bake for around 25 minutes, or until a cocktail stick inserted into the centre of the cake comes out clean. Remove from the oven. Take the cakes out of their tins and allow to cool completely on a wire rack.
Once cool, sandwich the two cakes together and cover with Betty Crocker Tempting Chocolate Icing.
To make the ganache, place the oat cream and chocolate in a saucepan over a low heat. Cook until the chocolate has completely melted and the mixture is well combined, stirring frequently. Remove from the heat.
To check the ganache is the right consistency to make the drip topping, drip a spoonful down the side of your bowl. Allow the ganache to cool until it is the consistency you want. [Note: I made my ganache in advance and stored it in the fridge, then re-warmed it in the microwave when I was ready to make the drip topping. This made it easier to get the correct consistency].
Drip spoonfuls of ganache down the sides of the cake, forming a ring all the way around the top of the cake. Spread any remaining ganache over the top of the cake. Push your chocolate buttons, white chocolate pieces and Gnawbles into the ganache and down slightly into the cake if needed. Sprinkle with gold glimmer sugar.