Special equipment: One 9-by-4-inch springform pan
Combine the sour cream and sugar in a large bowl and beat with an electric mixer on medium-high until smooth, about 3 minutes; scrape down the sides of the bowl as needed. Add the heavy cream, vanilla and lemon zest and continue to beat, starting on medium speed and increasing to medium-high, until stiff peaks form, about 2 minutes.
Toss the strawberries and blueberries together in a medium bowl.
Fill a resealable plastic bag with 1/4 cup of the whipped cream mixture and snip one of the corners off. Pipe the mixture around the bottom edge of a 9-by-4-inch springform pan (this will act as the glue for your lady finger edge). Push 21 of the lady fingers into the cream, standing them up with the pretty sugar-coated side facing out,. Now line the bottom of the pan with about 8 lady fingers, broken into pieces as needed, to make an even layer. Top with a third of the whipped cream mixture and spread out evenly. Scatter a third of the berries on top of the cream. Repeat the layering with another 8 lady fingers, a third of the whipped cream mixture and a third of the berries 2 more times for a total of 3 layers.
Loosely wrap the cake with plastic wrap and refrigerate for 12 hours or overnight to soften the lady fingers.
When ready to serve, unlock the pan and remove the collar from around the cake. Scoop portions into bowls with a large spoon.
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