Lemon Soufflé Pudding Cake

Cake on top, pudding on the bottom, tart all the way through.

4 Tbsp. (½ stick) unsalted butter, room temperature
¾ cup granulated sugar
1 cup buttermilk
½ cup crème fraîche or sour cream
2 tsp. lemon zest, finely grated
½ cup lemon juice, freshly squeezed
3 large eggs, separated
1/3 cup all-purpose flour, unbleached
¼ tsp. kosher salt

To Serve:
confectioners' sugar
fresh blackberries

Preheat oven to 325°F (165°C/140°C fan, Gas Mark 3). Butter a shallow 1½-2 qt baking dish, then sprinkle with granulated sugar.

Stir buttermilk, crème fraîche, lemon zest, and lemon juice in a medium bowl just to combine and set aside.

Using an electric mixer on high speed, beat 4 Tbsp. butter and ¾ 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.

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).

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.

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.

Serve pudding cake dusted with confectioners' sugar and topped with blackberries, if desired.

Makes 8 servings.

Make Ahead: Pudding cake can be baked 1 day ahead. Cover and chill. Bring to room temperature before serving.