Preheat oven to 350 degrees. Prepare 3 8-9 inch cake pans.
Filling: Put berries in pot, cover with water (just enough for berries to float)add sugar. Simmer on low for 20 min. or until juice is released, you don't want to cook the berries just get the juice from them. Cool completely.
Mix 2 whole eggs and 2 yolks with oil, sour cream, and sugar Add cake mix and water. Mix for 2 min.
Beat egg whites until stiff, fold into batter. Pour batter into pans. Bake 10-12 min or until done. DO NOT OVER BAKE ( it will dry the layers out )
Topping: Pour cream into large bowl ( I put my bowl and beaters in the refrigerater so they are ice cold, your cream will whip faster if everything is cold.) Add sugar and vanilla beat until thick and has stiff peaks.
STACKING the CAKE: Put 1st. layer on cake plate, poke holes in cake with a fork.
Spoon blackberry juice over layer and cover with berries,( single layer of berries) Add 1/3 of whipped cream on top of berries, add next layer and do the same. Top layer I cover w/berries and juice but I only put the whipping cream around the edge so you can see the blackberries.
This picture is of my cake after I went to move the cake to take a picture and tipped it and the top 2 layers went on the counter, lol Glad I had extra whipping cream.