How to Make It
Preheat the oven to 350°. Coat two 9-by-2-inch round cake pans with vegetable oil spray and line the bottoms with parchment paper. Lightly spray the paper. In a large bowl, sift together the flour, sugar, cocoa, baking soda and salt.
Pass the blackberries through a fine strainer set over a bowl; you should have 1 cup of puree. Whisk in the buttermilk, oil, vinegar and vanilla. Pour the blackberry mixture into the dry ingredients. Divide the batter evenly between the prepared pans and bake for about 40 minutes, or until the cakes pull away from the sides and the tops spring back when pressed. Let cool on a rack for 10 minutes, then unmold and cool completely.
In a large heatproof bowl set over a saucepan of simmering water, whisk the egg whites with the granulated sugar and cream of tartar until the sugar dissolves and the whites are hot to the touch, about 5 minutes. Transfer to a standing mixer and beat at high speed until the meringue cools to room temperature and is very thick, about 15 minutes. Stir the blackberry preserves until smooth, then gently fold 1 cup into the meringue.
Using a serrated knife, split the cakes in half horizontally. Thinly spread the remaining 1/2 cup of preserves on 3 of the cut cake layers. Set one of the layers on a large plate, preserves side up. Spread one-third of the meringue on top and press one-third of the blackberries into the meringue. Repeat with the 2 remaining preserves-spread cake layers and the remaining meringue and blackberries. Top with the final cake layer and let stand at room temperature for 1 to 3 hours. Just before serving, sift the confectioners' sugar over the top of the cake and garnish with the additional blackberries.