Separate egg yolks from egg whites.
In a bowl, beat the egg whites using a mixer at low speed for 1 minute or until frothy.
Sprinkle the cream of tartar on top and continue beating until soft peaks form.
Gradually add ¼ cup of the granulated sugar, a tablespoon at a time, while continuously beating. Once all sugar is added, turn up the speed to medium and continue beating the meringue until it forms stiff glossy peaks.
In a large bowl, cream together the egg yolks, the remaining ¼ cup sugar, milk, and vanilla extract until it becomes light in texture and color.
Sift the flour, cornstarch, and baking powder and add to the egg yolk mixture. Mix well.
Gently fold in a third of the meringue into the egg mixture and then add the rest and fold until well combined.
Pour the batter into a 15x10x1-inch pan lined with wax or parchment paper.
Bake in a 375 F oven for about 15 minutes or until the cake is golden.
Remove from the oven and loosen the edges of the cake. Transfer the cake including the wax paper to a rack and allow to cool.
Dust a wax or parchment paper with powdered sugar and invert cake onto the paper. Gently peel the other wax paper off the cake.
Beginning with the narrow side, roll the cake up together with the wax paper. Let it cool down completely, seam side down, for 10 to 15 minutes.
Once the cake has cooled, gently unroll and spread cake with softened margarine and sprinkle with granulated sugar and re-roll.
Cut into 1-inch thick slices to serve.