Put the sugar into a medium saucepan and heat gently, swirling the pan occasionally, until melted to a dark caramel. Reduce the heat to low and add 250ml water (it will spit, so stand back). The caramel will solidify but continue to stir over low heat until no lumps remain and you have a thin liquid caramel. Set aside to cool to room temperature, stirring occasionally.

Preheat the oven to 170 degrees Celsius. Grease an 18cm round cake tin and line the base with baking paper. Cream the butter and vanilla extract in a large bowl until the butter becomes slightly pale.

In a separate bowl, whisk the eggs until well combined. Add the eggs, condensed milk and caramel to the creamed butter and whisk to combine. Sift together the flour and bicarbonate of soda and use the whisk to gently stir into the butter mixture. Don't worry that the butter doesn't mix in evenly - this is how it should be.

Pour the cake batter into the tin and leave for a minute or two.

Bake for 50 minute or until the cake is springy to a gentle touch in the centre. Leave to cool in the tin, then run a knife around the edge before turning out.