Pre-heat the oven to 180° C.
Grease and line a 7 inch springform tart tin with nonstick baking paper. Combine the ricotta with 3 egg yolks and 1 egg white in a large mixing bowl. Whisk until smooth, then add the vanilla extract, lemon zest, sugar and orange blossom water if using.
Mix well again and set to one side. Whisk the remaining egg whites with a pinch of salt in a large mixing bowl until stiff peaks form. Fold the egg whites gently into the ricotta mixture.
Brush the tart tin with the melted butter, then spoon the mixture carefully on top, smoothing gently with the back of a spoon. Bake for 25-35 minutes until a knife comes out clean when inserted into the centre of the cake.
Remove from the oven and allow to cool in the tin before turning out and transferring to a serving plate. Serve warm or cold.