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1
Preheat oven to 325u00b0. Butter a shallow 1 1/2-2-quart baking dish, then sprinkle with granulated sugar. Stir buttermilk, creme fraiche, lemon zest, and lemon juice in a medium bowl just to combine and set aside.
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2
Using an electric mixer on high speed, beat 4 tablespoons butter and 3/4 cup granulated sugar in a large bowl until well combined, about 4 minutes. Add egg yolks 1 at a time, beating to blend between additions and scraping down sides of bowl as needed.
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3
Reduce mixer speed to low and, with motor running, add reserved buttermilk mixture in 3 additions, alternating with flour in 2 additions, beginning and ending with buttermilk mixture (batter will be extremely loose).
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4
Using an electric mixer with clean, dry beaters, beat egg whites and salt on high speed in a medium bowl until soft peaks form. Fold egg whites into pudding cake batter in 3 additions, just barely blending after each addition.
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5
Scrape pudding cake batter into prepared baking dish and place on a rimmed baking sheet. Bake until top is puffed and golden brown and center barely jiggles, 35-45 minutes. Transfer baking dish to a wire rack and let pudding cake cool at least 30 minutes.
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6
Serve pudding cake dusted with powdered sugar and topped with blackberries, if desired.
