Salted caramel celebration cake

Ingredients

• 750 gram unsalted butter, chopped
• 3 cup (660 grams) firmly packed brown sugar
• 2 teaspoon sea salt flakes
• 3/4 cup (180 millilitres) water
• 3 380 gram jars caramel spread (see tips)
• 3 1/2 cup (525 grams) plain flour
• 2 1/2 cup (375 grams) self-raising flour
• 6 eggs, beaten lightly
• 16cm round wooden cake board
• 20cm round wooden cake board
• 25cm round wooden cake board
• 6 thick wooden skewers
• Salted caramel buttercream
• 250 gram unsalted butter, softened
• 1/2 teaspoon salt flakes
• 1 1/2 cup (250 grams) icing sugar, sifted
• 380 gram jar caramel spread