Depending upon the number of guests you wish to serve, you can bake this in either a 7 and 1/2" or in a 10"springform pan.
Preheat your oven to 300 degrees.
For the 10" springform pan:
1 cup egg whites, room temperature (7 or 8, depending on size)
2 cups granulated sugar
1 teaspoon cream of tartar
1 Tablespoon apple cider vinegar
1 teaspoon vanilla extract
Place egg whites in the bowl of a standing mixer fitted with the whisk attachment. Beat until foamy then add cream of tartar and vinegar, beating until soft peaks form. Increase speed to high and add the sugar about a tablespoon at a time. The sugar needs to incorporate gradually- be patient. When the meringue is very thick and looks like marshmallow, turn the mixer down to low, add your vanilla and give it a few more turns. It is now ready to fill your springform pan.
Make sure you place the filled pan on a parchment lined baking sheet. Bake in pre-heated 300 degree oven for one hour. After the torte has baked for an hour, turn the oven off. Without opening the oven door, leave the torte in the oven for an additional hour and a half. This allows it to cool down gradually. Remove from the oven and place on a cooling rack. As it cools, the torte will fall. Before serving, remove from springform pan and cover with lightly sweetened whipped cream and fresh berries. You can also split the torte in half (it's very fragile) and fill it with cream and berries, then cover the top and sides with additional cream. Refrigerate until serving time.
For the 7 and 1/2" springform pan
1/2 cup egg whites, room temperature
1 cup granulated sugar
1/2 teaspoon cream of tartar
1 and 1/2 teaspoon apple cider vinegar
1/2 teaspoon vanilla extract
Follow above directions, adjusting baking time to one hour in the pre-heated 300 degree oven, then turn the oven off and let it cool in the oven for an additional hour.