- Beat the butter and icing sugar together until smooth.
- Push the hard boiled egg yolk through a sieve and stir the raw egg yolk and vanilla into it. Add this to the butter mixture and stir until blended.
- Add the flour and salt to the butter mixture and stir until blended. Shape the dough into a disc (it will be very soft), wrap in plastic and chill until firm, about 2 hours.
- On a lightly floured work surface, gently knead the dough just a little soften, then roll it out to a circle about 12 inches across and ¼-inch thick. Carefully lift this and line a 9-inch removable-bottom fluted tart pan, press it into the bottom and sides and trim away any excess dough. Chill the tart shell for 30 minutes.
- Preheat the oven to 325 ºF (160 ºC). Place the chilled tart shell onto a baking tray and dock the bottom of the pastry with a fork. Bake the tart shell for about 20 minutes, until the edges just begin to brown. Cool the tart shell to room temperature.
- For the filling, whisk the eggs, egg yolks, sugar, lemon zest and lemon juice together in a metal bowl and place this over a pot of gently simmering water. After a minute, add the butter and continue to whisk until the curd has thickened, about 5 minutes. Remove the curd from the heat, strain and whisk in the sour cream. Pour the warm curd into the cooled tart shell and chill, uncovered, until set, about 2 hours.