Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Line and grease 4 x 20cm round cake tins.
Place each box of Devil's Food Cake Mix into separate large bowls and mix each with 3 eggs, 120ml oil, 230ml water. Whisk (by hand or electric mixer) each for 2-3 minutes, until smooth and creamy.
Divide the cake mixtures evenly into the 4 greased and lined cake tins.
Bake in the centre of the oven for 18-20 minutes or until a rounded knife inserted into the centre of the cake comes out clean. Cool in the tins for 10 minutes. Run a knife around edges of cakes to loosen. Remove from tins and transfer to the cooling rack; cool completely, about 1 hour.
Make the salted caramel sauce by placing the condensed milk, brown sugar, butter and golden syrup into a large saucepan and place over a low heat. Bring to a gentle simmer, stirring constantly and cook for 2 minutes, being careful so it doesn’t stick to the base of the saucepan, until thickened and smooth. Stir in the salt and remove from the heat. Pour into a large bowl and leave to cool for 1 hour.
Make the salted caramel cheesecake filling by beating the cream cheese in a medium bowl, with 450g of the salted caramel sauce, stir until smooth and well combined.
Use a serrated knife to level the tops of cakes, if necessary. Place one layer, cut side down, on a platter or serving plate. Top with one-third of the cheesecake filling. Repeat with the remaining cake layers and filling, finishing with a cake layer. Chill for 15 minutes.
In large bowl, beat the icing with 200g of salted caramel sauce until smooth and well combined. Spread half of the buttercream mixture over the top and sides of the cake in a thin layer to make a crumb coat. Chill for 15 minutes. Repeat, spreading the top and sides of the cake with a second layer of the buttercream.
In small bowl, mix chopped pecans and crushed pretzels. Press the mixture around the bottom and 2 inches up the side of base of the cake. If desired, reserve some of mixture for the top of the cake. Chill until ready to serve. If desired, warm any remaining salted caramel sauce and drizzle over each slice when serving.