Wash orange hot and dab dry. Rub off the peel. Halve the orange and squeeze. Mix flour, yeast, sugar and 1 1/2 teaspoon salt in a mixing bowl, press a depression in the middle. Melt butter, add orange zest, 240 ml water and 4 eggs to the bowl. Mix everything with some flour from the rim. Cover with foil and chill overnight.
Chop the nuts and pistachios and roast them in a pot without fat until golden brown. Add condensed milk and 1 pinch of salt, stir and allow to caramelize over low heat. Add orange juice and simmer for about 6 minutes while stirring. Let the caramel-nut mixture cool down.
Knead the dough and roll out into a rectangle (approx. 38 cm x 50 cm) on a floured work surface. Spread the caramel-nut mixture evenly on the dough. Roll up the dough from the long side. Cut the yeast strand with scissors in the middle to approx. 2/3 approx. 20-25 times. (This creates many individual dough segments). Fold the first dough segment to the left, fold the second dough segment to the right and place the following dough segment centrally over the first two dough segments. Repeat this sequence until the plait is completely folded.
Whisk 1 egg and milk. Spread the yeast plait with it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 20-30 minutes. Take the yeast plait out of the oven and let it cool down. The yeast plait tastes best freshly baked.