Source: Local Flavors by Deborah Madison
Preheat oven to 375F. Grease an 11 inch tart pan. Cream together butter, ½ cup sugar and salt. Mix in the ½ vanilla, 2 eggs and the white of the third egg. Mix until smooth. Add in flour until just combined. Scrape batter into the tart pan, smooth out, making it slightly higher around the rim of the pan.
Mix the remaining egg yolk with the crème fraiche or sour cream and ½ teaspoon of vanilla. Pour this over the batter and spread around. Place berries on top until completely covered and sprinkle with 1 teaspoon of sugar. Bake until the custard is set, about 40 minutes. Let cool at least 30 minutes before serving. Dust with confectioners’ sugar just before serving.