In a large bowl, whisk together 2/3 cup of the granulated sugar and the flour. Add the lemon juice, zest, and buttermilk and whisk until smooth.
In a separate bowl, whisk together the butter and the egg yolks. Stir the egg yolk mixture into the buttermilk mixture.
Using the whisk attachment of a mixer, whip the egg whites until frothy. Sprinkle in the remaining 1/4 cup granulated sugar and whip until soft peaks form, about 1 minute. Fold the egg whites into the egg yolk and buttermilk mixture. The batter should be smooth and thick.
Pour the batter into an 8-inch square baking dish (or see note above if you prefer individual ramekins) and place the dish in a water bath with the water halfway up the sides of the dish. Bake 23 to 30 minutes, until slightly brown and beginning to crack but still jiggly. The smaller the baking dish, the less time will be needed to reach this stage. Cool to room temperature.
Make the chantilly cream
Whip together the cream, vanilla, and the 2 tablespoons of powdered sugar in a bowl until fluffy. Keep chilled until needed.
To serve
Turn the cake out onto a platter. (Individual ramekins can be left intact or overturned onto low, rimmed soup dishes.) If desired, spoon on the mashed berries, sprinkle with powdered sugar, and drizzle with Cointreau. Finish with a dollop of cream and, if desired, another dusting of powdered sugar. Originally published June 22, 2006.