Preheat the oven to 302º F (150° C). Line a square or round cake pan of about 8 inches (20 cm) with parchment paper.
In a large bowl, sift together the flour and salt.
Place the milk and butter in a medium heatproof bowl and microwave for 2 minutes to melt the butter and heat the milk, do it in intervals to make sure the milk doesn't boil. It can also be done on the stove in a pot, remember it is important not to let the mixture boil.
Immediately add the milk mixture to the bowl with the flour and beat with a hand whisk for 30 seconds until everything is well integrated.
Add the egg yolks with the vanilla extract and beat for another 30 seconds until smooth.
Meanwhile, place the egg whites in a large clean bowl and beat with an electric mixer for 1 minute until soft peaks begin to form. Add the sugar little by little with the vinegar and continue beating for another minute until stiff peaks form.
Add one third of the beaten egg whites to the bowl with the egg yolks and beat for a few seconds just until the mixtures are incorporated.
Now add the yolk mixture (lightened) to the bowl with the beaten egg whites and mix using a rubber spatula, making folding movements very carefully so as not to lower the whipped mixture.
Once the mixture is ready, pour it into the cake pan and place it in another rectangular mold with high walls filled one third full with hot water, but not boiling water.
Bake the cake in a double boiler for about 1 hour until it is lightly browned on top or when touched it bounces back a little.
When the cake is ready, remove it from the oven and let it cool in the cake pan for 10 minutes. Then turn it out, cut it and eat it when it has cooled down a little.