Put the flour and salt in a large bowl. Add the cold butter in small pieces to the flour and mix until crumbly with the mixer.
Mix the sour cream, egg yolk and vanilla pulp well together. Add to the flour mixture and knead until the ingredients are just mixed.
Cut the dough into three equal parts, press it into a flat disc and wrap it tightly with cling film. Chill for 2-3 hours or overnight.
Mix the sugar, walnuts and cinnamon together for the filling.
Preheat the oven to 175 °. Line baking trays with parchment paper.
On a lightly floured work surface, roll out the dough portions individually into a circle about 30 cm in size and 1.5-2 mm thick. Sprinkle evenly with 1/3 of the filling, press down lightly. Use a sharp knife or a pastry cutter to cut the circle into 16 pieces. Roll it up from the broad end and bend it into a crescent shape. Place with the point downwards with a little space on the prepared baking sheets.
Whip the remaining egg white until frothy. Spread evenly on each mini croissant with a brush.