In a large bowl, rub the butter into the flour until it resembles fine breadcrumbs.
Add egg yolk, soured cream, sugar and vanilla; mix well and knead together until smooth.
Form the dough into a ball; wrap in foil or clingfilm and chill for 1 hour.
Preheat the oven to 180 C / Gas 4 and lightly grease a baking tray.
On a lightly floured surface, roll out the dough until it is about 5mm thick. Cut into triangles. Place a teaspoon of jam on each triangle and roll up (starting from the widest part towards the point of the triangle) to form a crescent shape. Transfer to the baking tray.
Bake for 12 to 15 minutes until golden brown. Allow to cool on a wire rack. Dust with icing sugar.