Preheat the oven to 180°C (350°F) and grease a 20cm round cake tin.
In a saucepan over low heat, melt 100g of caster sugar until it caramelises, then pour it into the prepared cake tin, swirling to coat the bottom evenly. Allow it to set.
In a mixing bowl, whisk together the eggs, remaining caster sugar, and vanilla extract until well combined.
Heat the milk in a saucepan until warm but not boiling, then gradually pour it into the egg mixture, whisking constantly.
Sift in the flour and salt, then whisk until smooth and no lumps remain.
Pour the batter over the set caramel in the cake tin.
Place the cake tin in a larger roasting pan, then pour hot water into the roasting pan until it reaches halfway up the sides of the cake tin.
Bake in the preheated oven for 45 minutes or until the top is golden and a skewer inserted into the centre comes out clean.
Remove from the oven and allow to cool in the tin for 10 minutes before turning out onto a serving plate.
Serve the flan cake warm or chilled, slicing into wedges to enjoy.
