Line a large rimmed baking sheet with parchment paper. Set aside.
Combine the raisins, currants, orange juice and orange zest in a microwave safe container. Heat for 30 seconds on high power, stir and heat for another 15 seconds. Set aside to cool.
In a small bowl add 1/4 cup warm milk. (Make sure the milk is about 100°F - 105°F degrees using an instant read thermometer. If the milk is not warm enough the yeast will not bloom quickly. If the milk is too hot it will kill the yeast and not rise.) Add 1 teaspoon of granulated sugar to the warm milk and stir to dissolve. Sprinkle the yeast on top and stir lightly. Allow the milk and yeast to rest at room temperature until foamy, about 10 minutes.
In the bowl of a stand mixer, whisk together 3 ½ cups of the flour, ¼ cup of sugar, salt, cinnamon, nutmeg and allspice. Create a well in the middle of the flour mixture and pour in the bloomed yeast. Add the butter, eggs, egg yolk and the remaining ½ cup of warm milk. Using the paddle attachment, mix the ought until well blended. The mixture will be sticky. Add the currants, raisins, orange zest and orange juice. Mix until blended.
Switch the paddle attachment for a dough hook. Knead the dough on low speed while slowly adding the additional ¼ to a maximum of ½ cup of flour, one tablespoon at a time as needed to make a cohesive dough. Knead well after each addition until the dough is slightly sticky but doesn't stick to your fingers.
Form the dough into a ball and cover with plastic wrap or a clean kitchen tea towel. Allow the dough to rest in a warm location free from drafts, for 2 hours or until doubled in size. (I use my microwave as a proofing oven as it stays warm when the task light is on underneath.)
Press down the dough gently and turn out onto a lightly floured, clean work surface. Gently form the dough into a log. Cut the log in half and then each half into 8 equal size pieces. Form each piece of dough into a ball by pulling the sides down to the bottom pinching it together to seal. This will create a smooth top for each bun. Place the dough balls on the prepared pan about 1-inch apart. Gently cover the baking sheet with plastic wrap and allow the dough to rise again until the buns have doubled in size, about 45-60 minutes. (Again I put the dough in the microwave to keep warm and free from drafts. If placed in the microwave, there is no need to cover with plastic wrap)
Preheat oven to 400°F.