In a bowl, combine egg yolks, milk, vanilla extract, and lemon zest. Stir until well-blended.
Place in a double boiler and cook, stirring regularly, for about 30 to 40 minutes or until mixture thickens into a spreadable paste. Transfer custard into a bowl and cover with wax paper until ready to use.
Preheat oven to 350 F.
In a bowl, combine egg whites and cream of tartar. Using an electric mixer, beat at low speed.
As the egg whites start to turn opaque and begin to fluff into a thick foam, gradually add the sugar in small amounts and continue to beat while gradually increasing the speed of the mixer until stiff peaks form. The whipped whites will be smooth and glossy, with no sugar grains.
Line a 14 x 16 baking sheet with parchment paper and lightly grease the surface of the paper. Set aside.
Transfer meringue onto the prepared baking sheet and, using a spatula, spread evenly across into about ¼-inch thick.
Bake in the oven for about 20 minutes or until the meringue is set and the top has turned light brown. Remove from oven and allow to cool.
Using a fine-mesh sieve, light dust top of the meringue with powdered sugar.
Place another layer of greased wax paper and another baking sheet over the meringue. Gently invert the meringue onto the new baking sheet and peel the wax paper on top.
Spoon custard on top and spread evenly on the cooked meringue, leaving about ¼ without filling.
Starting on the filled side, carefully roll meringue into a log.
Cover both ends of the Brazo de Mercedes with wax paper and chill in the refrigerator for about 1 hour.
To serve, cut into desired thickness.