Preheat oven to 145°C fan-forced. To make caramel, stir 135gm sugar and 50ml water in a small saucepan over medium-high heat to dissolve sugar, then cook without stirring until very dark caramel (7-8 minutes; about 170°C on a sugar thermometer). Carefully pour into a 22cm round shallow cake tin and swirl quickly over base of tin. Set aside to cool and set (10-15 minutes).

Whisk eggs, yolks and remaining 150gm sugar in a bowl to combine. Bring cream and milk to a simmer in a saucepan over medium heat, then, stirring continuously, pour mixture over eggs and mix to combine, incorporating as little air as possible. Return to pan and stir continuously with a spatula until custard thickly coats the back of a wooden spoon (5-6 minutes; or until custard reaches 60°C on a digital thermometer).

Strain custard through a fine sieve into tin with caramel. Place in a roasting pan lined with a tea towel, then fill pan with boiling water to halfway up sides of the tin. Cover with foil and bake until custard is just set with a slight wobble in the centre (1 hour or until internal temperature of custard reaches 83°C-85°C; refill boiling water halfway through baking if evaporating). Cool custard in the water bath for 20 minutes, then remove and place on a rack to cool completely; gently release the sides of the custard with a wet finger once or twice as it cools, then refrigerate overnight to chill.

Run a knife around edges of tin to release, then invert custard onto plate and cut into slices to serve.