Pre-heat oven to 350F.
To make the meringue, combine egg whites and cream of tartar. Beat at low speed using an electric mixer until frothy.
Add the cornstarch and continue to beat at high speed until the meringue turns glossy. Gradually add the sugar. Beat for about 8 minutes until stiff peak or thick and heavy.
Transfer the meringue to a piping bag with large star tip. Using a circular, upward motion pipe the meringue into the cups. Leave a gap in the middle for the custard filling later.
Arrange cupcake cups in a baking tray. Bake for 15-18 minutes or until the tops light brown. Take out from the oven and let it cool.
To make the custard filling, use a small pot, combine egg yolks, condensed milk and all purpose cream. Turn the stove on to low heat and stir continuously.
Dissolve cornstarch with 2 tbsp water and mix it to the egg-milk mixture. Cook for 3 minutes or until it thickens. Transfer it to a bowl and let it cool.
Once the custard is completely cool down, transfer it to a piping bag with large star tip. Pipe the filling to the center of the meringue cups.
Using a sifter, top the brazo de mercedes cupcakes with powdered sugar.