Banana butterscotch syrup cake

Ingredients

• 125 gram (4 ounces) butter, softened
• 3/4 cup (165g) caster (superfine) sugar
• 2 eggs
• 1 cup mashed banana
• 3/4 cup (110g) self-raising flour
• 3/4 cup (110g) plain (all-purpose) flour
• 1/2 teaspoon bicarbonate of soda (baking soda)
• 3/4 cup (110g) hazelnuts, roasted, chopped finely
• 1/2 cup (110g) firmly packed light brown sugar
• 30 gram (1 ounce) butter, chopped
• 3/4 cup (180ml) water