Activate the Yeast: Heat the milk in the microwave for about 1.5 minutes until it reaches 140°F–145°F. Stir in the white sugar, which will drop the temperature to the ideal 120°F–130°F range. Stir in the instant yeast and let it sit for 5 minutes until foamy. (If it doesn't foam, start over with new store-bought yeast!)
Mix the Dough: In the bowl of a stand mixer, whisk together the bread flour and salt. Pour in the foamy yeast mixture, 1 whole egg, 1 egg yolk, and the dark rum. Using a dough hook, mix on low speed until a rough ball forms, then increase to medium speed for 2 minutes.
Knead in the Butter: With the mixer running on medium speed, add the softened butter one cube at a time, waiting until each cube is fully incorporated before adding the next. Knead for 5 to 7 minutes until the dough is smooth, elastic, and pulls away from the sides of the bowl.
Add Raisins & First Rise: Remove the dough from the mixer and gently fold in the raisins by hand to avoid crushing them. Cover the bowl with plastic wrap and a warm kitchen towel. Let the dough rise in a warm, draft-free spot for 1 to 2 hours, or until doubled in size.
Shape and Braid: Punch down the risen dough and divide it into 3 or 4 equal portions (depending on your braiding preference). Roll each portion into a 15-inch long rope. Pinch the top ends of the ropes together, braid them tightly, and pinch the bottom ends to seal. Tuck both ends slightly underneath the loaf for a clean finish.
Second Rise: Carefully transfer the braided loaf to a parchment-lined baking sheet. Cover it lightly with a towel and let it rise for 45 minutes until puffy. While the bread rises, preheat your oven to 350°F (177°C).
Glaze & Decorate: In a small bowl, whisk together the leftover egg white, 1 whole egg, and 1 tablespoon of milk. Use a pastry brush to apply the egg wash generously all over the risen loaf. Sprinkle heavily with sliced almonds (or amount to your liking).
Bake: Bake the bread for 15 minutes. Rotate the pan 180° to ensure even baking. (If the top is browning too quickly, loosely tent the loaf with aluminum foil). Bake for an additional 10 minutes, or until golden brown and hollow-sounding when tapped. Let cool on a wire rack for at least 15 minutes before slicing.