To a large-sized bowl (or stand mixer bowl, if using), add two cups of sourdough starter.
To the large bowl, add the granulated sugar, orange zest cinnamon, cloves, and allspice. Set aside.
In a medium-sized bowl, add 2 eggs and 1/4 cup honey, and whisk together. Next, add milk and 1 cup water and whisk together well.
Add the mixture in medium bowl to the large bowl and whisk everything together until completely incorporated.
Begin adding one cup of flour at a time into the wet mixture in the large bowl. You want a wet sticky dough, which will usually take adding the total 5 cups of flour.
Mix well after each addition of flour. You can do this by hand with a wooden spoon or Danish dough whisk. Alternatively, you can do this using a stand mixer with the dough whisk attachment.
Mix the dough until it begins to pull away from the sides of the bowl. Approximately 5-10 minutes.
Cover the bowl, and allow it to rest for 20 minutes.
Add salt and currants to the bowl and mix well until completely incorporated. Approximately 1 minute.
If using baking soda, add it along with salt and currants. (Please see RECIPE NOTES.)
Add butter to the bowl and mix well until completely incorporated. Approximately 1-2 minutes.
Transfer the dough to a floured board and knead for approximately 15-20 minutes until it is no longer sticky but very smooth. If you are using a stand mixer, this will take approximately 10 minutes.
Grease the large mixing bowl with olive oil. (If you are using a stand mixer, remove the kneaded dough from the mixer bowl and grease the bowl. Then return the dough to the greased mixer bowl.)
Place the kneaded dough back into the greased large mixing bowl. Cover the bowl and allow the dough to rise or double in bulk for 1 hour.
While the dough is rising, add the orange juice and 1 teaspoon of honey to a small saucepan. Mix well and place on medium-high heat, bringing the mixture to a boil. Immediately turn down the heat to low and allow the juice and honey mixture to simmer until reduced by half. Remove from the heat and set aside, but cover the saucepan to keep the mixture warm.
Next, heavily grease a 9"x13" baking dish with 1 tablespoon of butter.
After one hour, punch down the dough and divide the dough into 24 equally-sized pieces. (See video.)
Roll each piece of dough into a ball and place it into the greased baking dish.
Loosely cover the baking dish containing the dough balls with plastic wrap and then a dish towel and place the dish in a warm place to allow the dough balls to rise to double in size. Approximately 1 hour.
Once the dough balls have risen sufficiently after approximately 1 hour, use a sharp knife or a lame (a French baker's tool) to make two cuts to form a cross on the top of each dough ball. Set aside.
Preheat the oven to 400°F (204°C).
In a small bowl, whisk one egg and 1 tablespoon of water together. Brush this mixture over the tops of the risen dough balls using a pastry brush or the back of a spoon.
Place the baking dish on the middle rack of the preheated oven and bake for approximately 15-18 minutes until the hot cross buns are golden brown.
Once hot cross buns are finished baking, remove them from the oven and place the baking dish on a heatproof surface. While still in the baking dish, immediately brush the juice honey mixture on the tops of the buns. This works best using a silicone pastry brush, but if you do not have one, you can use the back of a spoon.
Leave the hot cross buns in the baking dish to cool.
While the hot cross buns are cooling, make the icing by mixing together the powdered sugar and 1 teaspoon of water (milk can also be used in place of the water) in a bowl and then transfer to a pastry bag or plastic bag. If you use a plastic bag, you will need to cut off the tip of the bag. (See video.)
Once the hot cross buns have completely cooled, pipe the icing in a cross pattern on top of each bun.
Your hot cross buns are now ready to serve or can be covered loosely and served later in the day at dinner time.
If any hot cross buns remain after dinner, cover them loosely and leave at room temperature. They are best if they are all consumed by the next day.
You can make the hot cross buns a day ahead and refrigerate them. Make sure they are wrapped well and take care not to disrupt the icing. To serve, remove the hot cross buns from the refrigerator, uncover, and allow them to return to room temperature before serving.
Hot cross buns may also be frozen for 2-3 months if wrapped well. However, you might want to refrigerate them first to allow the icing to harden before wrapping them for the freezer. When ready to eat, remove the buns from the freezer, unwrap them, and allow them to defrost and return to room temperature before serving.