In a bowl, beat the egg whites using a mixer at low speed for a minute or until frothy. Sprinkle the cream of tartar on top then beat until soft peaks form.
Gradually add half 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 sugar, vanilla extract, and milk until it becomes light in texture and color.
Sift the all-purpose flour, cornstarch, and baking powder then add to the egg yolk mixture.
Gently fold in a third of the meringue into the egg mixture and then add the rest and add the food coloring mixed until well combined. Do not overmix.
Pour the batter into a pan lined with wax or parchment paper. Spread it evenly all over the pan. Bake in a preheated 375°F oven for about 15 minutes or until the cake is golden.
Remove from the oven and loosen the edges of the cake. Let it cool a bit. Dust the cake with powdered sugar and invert the cake onto the paper. Gently peel the other wax paper off the cake. Roll the cake up together with the wax paper. Let it cool down completely for 10-15 minutes.
In a cold bowl, beat the whipping cream using a mixer at medium speed and gradually add the powdered sugar until soft peaks. Fold in the mashed avocado until everything is well blended.
Once the cake has cooled, gently unroll the cake and spread the avocado cream filling over it. Carefully re-roll the cake and let it chill until ready to serve.