Put the butter in your mixing bowl and use the wrapper to lightly grease an 11-inch tart pan. Add the 1/2 cup to the butter and cream it together. Add 1/2 tsp vanilla and 2 whole eggs plus 1 egg white, saving the other yolk to mix with the sour cream. Beat together, scraping the sides of the bowl down periodically. Add the flour and mix in but don't overmix. Pour the batter into the tart pan and spread it around and up the sides of the pan a bit.
Mix the egg yolk and sour cream with 1/2 tsp vanilla. Pour over the batter and spread it out to the edges.
Drop the berries onto the custard so it's as covered as possible. Sprinkle a teaspoon of sugar over the top.
Bake about 40 minutes, until the edges are lightly browned and custard is set in the middle.