Preheat oven to 160°C/140°C fan forced. Place a tea towel in the base of a roasting pan.

Place sugar and water in a saucepan over low heat and stir for 3-5 minutes or until the sugar dissolves. Increase heat to high and bring to the boil. Cook, without stirring, brushing down side of pan with a brush dipped in water, for 5-8 minutes or until light golden – don't let it get too dark. Remove from the heat and swirl the pan to cool the toffee and allow the bubbles to settle – the residual heat will continue to colour the toffee. Pour the toffee into a deep 19cm (base size) round cake pan. Set aside to cool and harden.

Place eggs and egg yolk in a bowl. Add sweetened condensed milk and whisk until just combined. Add milk and vanilla, and whisk until well combined. Pour into a sieve set over a large jug. Pour over the back of a spoon over the toffee layer. Place the cake pan inside the roasting pan and cover with foil. Add enough hot water to the roasting pan to reach halfway up the side of the cake pan.

Bake for 1 hour 10 minutes or until a knife inserted into the centre comes out clean (the custard should still be very wobbly, but you shouldn’t see ripples appear on the surface). Remove cake pan from the roasting pan. Set aside for 2 hours to cool. Place in the fridge for 6 hours, or overnight, to chill. Run a sharp knife around the inside edge of the pan and turn onto a serving plate to serve.