1 For the filling: Bring the milk to a boil, stir in the ground poppy seeds, sugar and butter. Mix to form a thick, homogenous paste, then cool.
2 For the dough: Mix the pastry flour into a bowl with the sugar and vanilla sugar then and make a well in the middle.
3 Stir the yeast into the milk and pour into the well. Mix with some flour from the edge, cover, and let rise for 15 minutes in a warm place. Then add the butter, egg, rum and orange zest and knead to form a dough. Knead well for about 10 minutes until the dough is elastic and no longer sticks to the bowl. Cover and let rise for 45 minutes.
4 Line the loaf pan with parchment paper. Knead the dough again on a floured work surface and then roll out into a rectangle about 30 x 60 cm (approximately 12 x 24 inches). Along the short side of the rectangle, spread a 10 cm (approximately 4 inches) wide strip of the poppy seed filling and then fold the remaining dough over to cover it. Repeat this procedure 4 times in order to fill and shape the dough. Place the shaped dough into a buttered loaf pan, cover, and let rise for another 20 minutes.
5 Preheat the oven to 180°C (approximately 350°F).
6 Bake the cake in the preheated oven for about 45 minutes. Then turn out onto a wire rack and cool.
7 For decorating: Mix the powdered sugar with 2 teaspoons lemon juice and brush the outside of the cake with it. Garnish with the walnuts and dried fruit and allow to dry overnight. To serve, slice into pieces.