Make the chocolate sponges: Preheat the oven to 160c.
Grease and line the bottom and sides of two 7 inch round cake tins.
Put all the sponge ingredients into a bowl and mix well. Evenly divide the batter between the 2 tins, and bake for 20 minutes. Allow them to cool in the tin for 10 minutes, then remove from the tin and transfer to a cooling rack.
Make the peanut butter sponge: Preheat the oven to 160c.
Put all the sponge ingredients into a bowl and mix well. Pour the batter into the cake tin and bake for 20 minutes. Allow it to cool in the tin for 10 minutes, then remove from the tin and transfer to a cooling rack.
Make the puppy chow: Empty the cereal into a large bowl.
In a saucepan over low heat, combine the chocolate chips, peanut butter, and butter. Stir occasionally until the mixture is melted. Pour the melted mixture over the cereal, and gently mix to coat. Transfer the chocolate coated cereal to a large clean bag and add in the icing sugar. Twist the bag closed and shake it really well until the cereal is coated. Add more icing sugar if needed.
Make the buttercream:In a mixing bowl, beat the butter using an electric mixer until fluffy. Add in the vanilla extract and icing sugar then beat for a few minutes until smooth.
To assemble: Place your first layer of chocolate cake on top of your cake stand. Put buttercream on top of the first layer and spread evenly. In a single layer, arrange puppy chow on top of the first chocolate layer. Place the peanut butter layer on next and spread on buttercream. Then arrange more puppy chow. Place the second chocolate layer on top of the others and spread on buttercream. Cover the entire cake in a crumb coat of buttercream. Place the entire cake in the freezer for fridge for 20 minutes to harden the crumb coat, then ice another layer. Use a piping bag to add a border around the bottom of the cake and pipe a few swirls on top of the cake. Top each buttercream swirl with a few puppy chow pieces.