Preheat the oven to 180°C (160°C fan forced). Line and grease 4 x 20cm round cake pans. Remove enclosed frosting sachet from cake mix box and reserve for another use.
Place each box of Devil's Food Cake Mix into separate large bowls and mix each with 3 eggs, 3/4 cup (180ml) water and 1/3 cup oil. Mix each bowl with electric mixer on LOW speed for 30 seconds, then on MEDIUM speed for 2 minutes (do not eat batter as it contains raw eggs.)
Divide the cake batters evenly into the 4 greased and lined cake pans.
Bake in the centre of the oven for 18-20 minutes or until skewer inserted into the centre of the cake comes out clean. Cool in pans 10 minutes. Run knife around edges of cakes to loosen. Remove from pan to cooling rack; cool completely, about 1 hour.
Make the salted caramel sauce. Place the condensed milk, brown sugar, butter and golden syrup into a large pan 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 pan, 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. Beat the cream cheese in a medium bowl, with 450g salted caramel sauce, until smooth and well combined.
Use serrated knife to level tops of cakes, if necessary. Place one layer, cut side down, on platter or serving plate. Top with one-third of the cheesecake filling. Repeat with remaining cake layers and filling, finishing with a cake layer. Chill for 15 minutes.
Make the Salted Caramel Buttercream. In large bowl, beat the frosting with 200g salted caramel sauce until smooth and well combined. Spread half of the buttercream 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 mixture around bottom and 2 inches up side of base of cake. If desired, reserve some of mixture for top of cake. Chill until ready to serve. If desired, warm any remaining salted caramel sauce and drizzle over each slice when serving.