Preheat the oven to 180°C and line a 9" springform baking tin with one large sheet of baking paper.
In a large bowl, beat the butter and sugar together until light and fluffy.
Whisk in the eggs, followed by the flour, vanilla and salt.
Spoon into your tin and smooth down flat. Bang the tin on the counter a couple of times to knock large air bubbles out.
Pour over the custard, smoothing it into an even layer with the back of a spoon.
Bake for about 45 minutes, or until puffy and a deep golden brown on top. Leave to cool before releasing from the tin, slicing and serving.
