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1
Preheat the oven to 350.
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2
Coat two 9-by-2-inch round cake pans with vegetable oil spray and line the bottoms with parchment paper.
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3
Lightly spray the paper.
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4
In a large bowl, sift together the flour, sugar, cocoa, baking soda and salt.
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5
Pass the blackberries through a fine strainer set over a bowl; you should have 1 cup of puree.
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6
Whisk in the buttermilk, oil, vinegar and vanilla.
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7
Pour the blackberry mixture into the dry ingredients.
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8
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.
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9
Let cool on a rack for 10 minutes, then unmold and cool completely.
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10
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.
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11
Transfer to a standing mixer and beat at high speed until the meringue cools to room temperature and is very thick, about 15 minutes.
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12
Stir the blackberry preserves until smooth, then gently fold 1 cup into the meringue.
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13
Using a serrated knife, split the cakes in half horizontally.
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14
Thinly spread the remaining 1/2 cup of preserves on 3 of the cut cake layers.
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15
Set one of the layers on a large plate, preserves side up.
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16
Spread one-third of the meringue on top and press one-third of the blackberries into the meringue.
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17
Repeat with the 2 remaining preserves-spread cake layers and the remaining meringue and blackberries.
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18
Top with the final cake layer and let stand at room temperature for 1 to 3 hours.
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19
Just before serving, sift the confectioners' sugar over the top of the cake and garnish with the additional blackberries.
