Place sour cream, sugar and blackcurrants in a blender and blend to a homogeneous mixture.
Place approx. 350 g flour and cold butter in a mixing bowl and work quickly into fine crumbles.
Add baking soda, vanilla and salt to the crumbles and mix.
Add the berry sour cream mixture and egg yolk and knead briefly and quickly to form a very soft, non-sticky dough, adding the remaining flour as required.
Divide the dough into two halves and chill in an airtight container for approx. 1 hour.
Lightly flour the work surface. First take one half of the dough out of the fridge, divide it into 10 equal pieces (38 - 39 g each in my case), shape each piece into a ball and gently flatten each ball into a circle. Cut the sides of the dough circle like a flower and pull the ends of the “petals” together. Make a small hollow in the center of each “flower” and draw lines on the “petals” with a knife.
Spread the flower cookies well apart on a baking tray lined with baking paper, fill the hollows in the middle with a small amount of jam and sprinkle some sesame seeds on top.
Bake the flower cookies in a preheated oven at 338 °F (170 °C) for approx. 20 minutes, covering them with baking paper after approx. 10 minutes to prevent them from browning.
Shape the second half of the dough into flower cookies and then bake them on a second baking tray.