1.Preheat oven to 150°C (130°C fan-forced). Grease deep 16cm, 20cm and 25cm round cake pans. Line bases and sides with a double layer of baking paper, bringing paper 3cm above side of pan.
2.In a very large saucepan, combine butter, sugar and salt and water; stir over low heat, without boiling, until smooth. Remove from heat; add caramel, whisk until smooth. Cool 30 minutes.
3.Whisk flours and eggs into caramel mixture, in batches, until smooth. Divide mixture evenly among the prepared pans. Bake smaller cakes 1 1/2 hours and large cake 1 3/4 hours or until skewer inserted into cakes comes out clean. Stand cakes in pans 15 minutes before turning, top-side up, onto a wire rack to cool.
4.Repeat Steps 1, 2 and 3 once more; you should have two of each size cake (six in total).
5.In a large bowl, beat butter and salt flakes with an electric mixer about 5 minutes or until pale and fluffy. Gradually add icing sugar, beating until mixture is light and fluffy. Gradually add caramel, while still beating, until mixture is well combined. Chill until firm. Whip buttercream until mixture is pale, fluffy and spreadable.
6.Using a serrated knife, trim any domes off tops of each cake. Split each cake crossways into two even layers. Place bottom layer of one of the 25cm cakes on the 25cm cake board. Spread with about 1cm-thick layer of buttercream. Place another layer of cake on top; continue layering with remaining 25cm cake layers and buttercream, not spreading the top layer with buttercream. Repeat this process with both the 20cm cakes and 16cm cakes, and their corresponding cake boards. Refrigerate each cake for 30 minutes.
7.Spread a little buttercream on cake stand; position 25cm cake with its board onto cake stand. Insert 3 wooden skewers into the centre section of cake. Mark the skewers with a pencil close to cake surface. Using kitchen scissors, trim skewers at the mark; discard pointed end. Insert trimmed skewers into the cake. Now position 20cm cake with its board on top of 25cm cake. Trim remaining skewers; repeat this process once more with 16cm cake.
8.Using a spatula or palette knife, coat entire cake with half remaining buttercream. Pipe remaining half of buttercream onto cake as desired to decorate.
To make your own caramel spread, pour condensed milk into a medium deep baking dish; cover with foil. Place dish inside a large baking dish; add enough boiling water to large dish to come halfway up sides of smaller dish. Bake at 220°C (200°C fan-forced) for 1 1/2 hours or until condensed milk is golden brown. Whisk caramel until smooth, cover; cool to room temperature. This makes one x 16cm, one x 20cm and one x 25cm round cake. You will need double this amount to make a three-tier cake. Our cake has two cakes of each size to make each tier. You will need approximately 5 batches of buttercream to decorate a three-tier cake.