I'm not sure why this is "Spanish" as opposed to another version, "Bavarian", which has more cream in it. This recipe comes from a Women's Division of Federated Farmers Cookbook, published in 1965.
600ml of milk ( 1 pint)
2.5ml vanilla essence
1 Save a small amount of the milk; put the rest in a saucepan together with the sugar and vanilla, and heat through.
2 Separate the egg whites and save for later. Beat the egg yolks with the remainder of the milk and add to the heating mixture.
3 Stir to ensure the sugar is dissolved and bring to near boiling point to thoroughly cook the eggs. Do not actually boil.
4 Take the saucepan off the heat. Dissolve the gelatine in 60ml of boiling water and stir into the hot custard.
5 Pour into a serving dish and put in the fridge to cool.
6 When mixture has cooled and almost set (it should be shivery at this point) thoroughly beat the egg whites until stiff and fold them in using a metal spoon. Return the mixture to the fridge to set properly.
This goes well with any poached fruit but nectarines poached with a vanilla pod would be delicious.