In a bowl, combine the sugar and lemon peel, if using, and mix well.

Warm the milk slightly and add the diced unsalted butter, stirring until melted.

Sprinkle the fresh yeast over the milk mixture, or add the dried instant yeast, and let it sit for a few minutes until frothy.

In a large mixing bowl, whisk together the egg yolk, strong white bread flour, and fine sea salt.

Pour the milk mixture into the flour mixture and mix until a dough forms.

Knead the dough on a floured surface until smooth and elastic, then place it in a lightly greased bowl and cover it with a cloth.

Allow the dough to rise in a warm place for about an hour, or until it has doubled in size.

Once risen, roll out the dough into a rectangle on a floured surface.

Spread the soft unsalted butter over the dough, then sprinkle with sugar and ground cinnamon.

Add the halved walnuts or mixed candied fruit evenly over the top.

Roll the dough tightly from one end to the other to form a log.

Twist the log into a spiral shape and place it on a baking tray lined with parchment paper.

Cover the kringle and let it rise for another 30 minutes.

Preheat the oven to 180°C (350°F).

Bake the kringle for about 25-30 minutes, or until golden brown.

While the kringle is baking, prepare the icing by mixing the lightly beaten egg whites with icing sugar until smooth.

Once the kringle has cooled slightly, drizzle the icing over the top.