Begin by mixing together the crushed biscuits, sugar and melted butter to form the biscuit base. Press this firmly into a 9" tart or pie shell and refrigerate for an hour while preparing the filling. For tips see my recipe here. Place the egg yolks in a heat proof bowl and set aside.
In order to make the manjar blanco, pour the condensed milk and evaporated milk into a saucepan. Cook over a medium heat, stirring occasionally until the mixture begins to boil. Then, stirring constantly continue to boil for about 5 minutes.
Removing the milk from the heat and begin to whisk the egg yolks. Whisking the yolks constantly, slowly pour about half of the milk mixture into the eggs. Pour this back into the saucepan with the rest of the milk and whisk, returning to the heat.
Continue whisking constantly for another 5 minutes until thickened to a pourable custard consistency. Remove from the heat, whisk in the vanilla extract and pour straight into your prepared pie crust.
Set the manjar filled tart aside while you prepare the port meringue.
Place the port and ½ cup sugar in a small saucepan over a medium heat, stirring until the sugar has dissolved. Continue to heat on medium, bringing the syrup to a boil.
Once the syrup begins boiling, stop stirring and allow to to boil for three minutes. Meanwhile, place the egg whites in a large mixing bowl and beat with a handheld electric mixer until soft peaks form.
After 3 minutes, remove the syrup from the heat and, while whisking the egg whites continuously, slowly pour the syrup down the side of the bowl into the meringue.
Continue whisking on a high speed until the meringue is glossy, cool and stiff peaked. Then, transfer to a piping bag to pipe or dollop onto the manjar blanco custard filling.
Allow to cool for at least half an hour at room temperature before refrigerating.
When ready to serve, sprinkle with ground cinnamon.