Place 1 cup (150g) of raisins in a bowl, cover with boiling water, and soak for 15 to 20 minutes. Drain through a fine mesh strainer and pat dry thoroughly with paper towels to remove excess moisture.
Gently heat 1 cup (240ml) of whole milk until it reaches 110°F (43°C) — it should feel comfortably warm but not hot to the touch. Set aside.
In the bowl of a stand mixer, whisk together the flour, instant yeast, sugar, cinnamon, allspice, nutmeg, salt, and orange zest until evenly combined.
Add the warmed milk, softened butter, eggs, and vanilla extract to the dry ingredients. Mix on low with the dough hook until a shaggy dough forms, then increase to medium speed and knead for 5 minutes until smooth and elastic.
Reduce mixer speed to low and add the drained raisins gradually, mixing for an additional 2 minutes until evenly distributed. The dough should be slightly tacky but not sticky to the hands.
Shape the dough into a smooth ball, place in a lightly greased bowl, cover with plastic wrap, and let rise in a warm place for 1 to 1.5 hours until doubled in size.
Turn the risen dough onto a lightly floured surface, punch down, and divide into 12 equal pieces (about 80-85g each). Roll each piece into a smooth ball using a cupped palm with gentle circular pressure, then arrange in a greased 9x13 inch pan in a 3x4 grid.
Cover the pan loosely and let the buns rise in a warm place for 45 minutes to 1 hour until puffed and touching at the sides. Meanwhile, preheat the oven to 375°F (190°C).
Whisk together 1/2 cup (60g) flour and 5 to 6 tablespoons of water until a smooth, thick paste forms. Transfer to a piping bag with a small opening (about 3mm).
Whisk together the egg and 1 tablespoon of milk, then brush generously over the tops of all risen buns. Immediately pipe straight lines of flour paste across all rows and columns to form a cross on each bun.
Bake at 375°F (190°C) for 18 to 22 minutes until deeply golden brown and the internal temperature reads 190°F (88°C). Tent loosely with foil in the final 5 minutes if tops are browning too quickly.
Simmer 3 tablespoons each of sugar and water in a small saucepan for 1 to 2 minutes to make a glaze, then brush generously over the hot buns immediately out of the oven. Cool in the pan for 10 minutes before serving.