For the Custard Filling
10 egg yolks, beaten
1 (14 ounces) can condensed milk
1 teaspoon vanilla extract
zest of 1 lemon
For the Meringue
10 egg whites
1 teaspoon cream of tartar
1 cup superfine sugar
¼ cup powdered sugar
For the Custard Filling:
In a double broiler (heatproof bowl over a heated pot of boiling water) using lowest heat, mix egg yolks, milk, vanilla extract and lemon zest. Cook, mix constantly, for approximately 30 to 40 minutes (until mixture thickens into a spreadable paste.) Turn off heat and place your custard into a bowl, cover with wax paper and set aside for later use.
For the Meringue:
In a large mixing bowl, add egg whites and cream of tartar. Using an electric hand mixer, beat at low speed. Once soft peak starts to form slowly add the sugar part by part and continue to beat. Increase your hand mixer speed too. Continue beating until it becomes hard peak. It may take longer than you know. By hard peak it means your meringue is no longer moving even if you invert your bowl, it won’t even slide or move at all. Just be careful because over beating of egg whites may cause your meringue to turn watery.
Line a baking tray with wax paper and oil the surface of paper. Place your meringue onto prepared baking sheet and using a spatula, spread evenly across into about ¼-inch thick. Bake in a 350 F oven for about 20 minutes or until meringue is set and top has turned to light brown.
Once cooked don’t be too excited. Turn off heat. Do not take it out immediately from the oven. Just let the oven doors open while it cools down. It may deflate your meringue with sudden change of temperature. It needs to cool down gradually and completely before you do the next step.
Use a sieve and gently dust the top with powdered sugar. Place another layer of greased wax paper and another baking sheet over meringue. Gently invert meringue onto the new baking sheet and peel the wax paper on top.
Spoon custard on top and spread evenly on cooked meringue, leaving about an extra space ¼ without filling. Just put enough filling so it won’t spill when rolled.
Starting on the filled side, carefully lift the paper so as not to touch it. Roll meringue into a log while holding the paper liner as a protection on top. You can use a baking comb to make the striped look on top.
Cover both ends of the brazo de mercedes with wax paper and chill in the refrigerator for about 1 hour.
Do not use old stock eggs, you need fresh eggs for better whipping. Egg whites should be at room temperature for about 30 minutes before beating.
Use a copper, stainless or glass bowl. Avoid aluminum bowls as they can turn egg whites to gray.
Use very clean and dry bowls and beaters. Fats or any residue can affect the egg’s volume. Make sure not even a drip of yolk gets into the whites. Or your egg whites will not form into a meringue. Add sugar slowly to the egg whites so they don’t lose volume.
Do not overbeat! You know they have reached stiff peaks when the whipped whites no longer slide around when the bowl is tilted. Your hint to stop is when you trash your bowl and the meringue doesn’t give any damn. Kidding aside, your meringue will not move even if you trash it in the air.
To prevent the egg yolks from curdling, make sure to cook the custard over low heat.