In the bowl of your stand mixer, combine butter, milk, flour, sugar, salt, cardamom (optional), and fresh yeast.

Attach the hook to the stand mixer and mix on low speed for 5 minutes to bring the dough together.

Increase the speed to medium/medium-fast and continue mixing for about 30 minutes to build strength in the gluten. To check readiness, stretch a piece of dough with wet hands—it should be able to get really thin and not tear.

Shape the dough into a rectangular-ish shape, wrap in plastic, and refrigerate for 1 hour.

While the dough rests, prepare the filling by mixing softened butter, sugar, vanilla sugar, and salt. Set aside (do not refrigerate).

Roll out the fully chilled dough on a floured surface. Make it about ⅓ inch or 1cm thick, and 1½ to 2 times longer than wide.

Spread the butter filling evenly over the dough, then zest a lemon over the top.

Sprinkle blueberries evenly over the filling and press gently into the butter.

Fold down the top third and fold up the bottom third of the dough.

Roll the dough again to about ¾ inch or 2cm thickness. Cut into long, skinny strips weighing 80-90g.

Tie each strip into a knot and place on a parchment-lined baking tray.

Cover with plastic wrap or a tea towel and let the knots proof until doubled in size (1-3 hours).

Once doubled, bake at 400°F (200°C) for 8-10 minutes until golden brown.

Immediately after baking, brush the buns with a thin layer of egg wash and let cool completely.

Mix lemon juice and powdered sugar to create the glaze. Adjust thickness to your preference.

Drizzle the glaze over the cooled buns.

Eat and enjoy!