Activate the yeast: Warm 1 cup milk in a small saucepan until lukewarm (105°F–110°F / 40°C–43°C). In a large bowl, mix 3 ⅔ cups all-purpose flour and ⅔ cup granulated sugar, then make a well in the center. Pour in one-third of the milk and add 2 ½ teaspoons active dry yeast. Stir gently with a fork and let it activate in a warm spot for about 10 minutes, until bubbly and doubled in volume.
Add 1 stick unsalted butter to the rest of the warm milk and let it melt. Finely grate the yellow part of the lemon zest.
Add 3 egg yolks, ½ teaspoon vanilla extract, 1 Tablespoon rum,½ teaspoon salt, and 1 Tablespoon lemon zest to the bowl with the activated yeast. Pour in the milk with the melted butter. Using a stand mixer fitted with a dough hook, knead the dough for about 10 minutes. Occasionally stop the mixer to scrape the dough down from the sides of the bowl.
While kneading the dough, coarsely chop 1 cup blanched almonds. When the dough is done, add the almonds and ⅔ cup raisins. Mix on low speed for about a minute to combine evenly.
Transfer the dough to a clean bowl, cover with plastic wrap, and let it rise in a warm spot. Due to its high butter content, the dough is heavier and may take about two hours or more to rise, depending on the temperature in your kitchen.
Turn the proofed dough onto a floured surface, sprinkle lightly with flour, and knead briefly. Divide it into nine equal pieces, weighing them for accuracy if desired. Roll each piece into a strand ¾–1 inch (2–2.5 cm) thick and equal in length.
Assemble the Vánočka: Braid four strands for the base layer, three strands for the middle, and twist two strands together for the top layer.
Start by braiding the four strands for the base and placing it on a parchment-lined baking sheet. Flatten the center slightly. Add the three-strand braid on top, then the two twisted strands. Tuck the ends neatly under the loaf.
Second rise: Let the braided Vánočka rise for another 30 minutes in a warm place.
Brush, decorate, and bake: Whisk 1 egg with 1 teaspoon rum and brush it generously over the entire loaf. Decorate the top with halved ¼ cup blanched almonds.
Bake the Vánočka in a preheated oven at 170°C for 20 minutes, then reduce to 150°C and bake for another 35 minutes. Cover with foil if it browns too quickly.
Once baked, allow the Vánočka to cool on the baking sheet before slicing. Optionally, you can sprinkle its top with 2 Tablespoons powdered sugar.