Preheat oven to 170 C / Gas 3. Place a 23cm heatproof pie dish into a deep roasting tin; pour boiling water into roasting tin to reach about halfway up the side of the dish.
Pour 1 teaspoon water and 100g caster sugar into a heavy saucepan; place over medium heat and constantly swirl the sugar-water mixture in the pan over the heat until sugar dissolves, melts, and turns into a golden-brown caramel syrup, about 10 minutes. Watch carefully, the sugar burns easily. Pour melted caramel into the warmed pie dish and gently tilt pie dish to coat the bottom with caramel.
Whisk egg yolks, sweetened condensed milk, evaporated milk, 150g caster sugar and vanilla extract in a bowl until mixture is creamy and thoroughly combined. If mixture is foamy, skim foam off with a piece of kitchen roll. Gently pour custard over caramel in pie dish. Cover baking pan with foil.
Bake in the preheated oven until custard is set but still jiggles slightly when moved, 45 minutes to 1 hour. Remove foil and place tin with and flan in the fridge to chill, about 2 hours.
Remove pie dish; run a knife around the edges of the flan to loosen from pie dish. Invert flan onto a rimmed serving platter to release custard and let caramel sauce flow over the flan.