In a large mixing bowl, combine the all-purpose flour, protein powder (if using), granulated sweetener, ground cinnamon, nutmeg, allspice, salt, and instant yeast.
Warm the milk in the microwave for about 30 seconds until slightly warm. Add the warmed soy milk to the dry ingredients. If the dough appears too dry, gradually add additional milk, 15ml at a time, until a slightly sticky dough forms.
In a separate bowl, mix together the egg yolk, yogurt, and vanilla extract until well combined. Add this mixture to the dry ingredients and mix thoroughly to form a sticky dough.
Optional: Cover the dough with a cloth, cling wrap, or a shower cap and let it rest for 10 minutes.
Dust your work surface with flour and knead the dough for 10-15 minutes by hand or using a stand mixer on low to medium speed. Incorporate the chocolate chips or chopped chocolate while kneading.
Place the kneaded dough into a greased bowl, cover it well, and let it rise in a warm, draft-free place for 1-3 hours, or until doubled in size. Note: The addition of protein may slightly slow down the rising process.
Once the dough has doubled in size, punch the air out and roll it into a log shape. Cut the dough into 9 equal parts.
Flatten each piece of dough and form it into a ball. Place the dough balls onto a greased baking tray. Cover the dough balls well and let them rise for another 1-1.5 hours.
Preheat your oven to 180°C (350°F). In a small bowl, mix together the all-purpose flour and water to form a paste for the crosses. Pipe the crosses over the risen dough balls.
Bake the hot cross buns in the preheated oven for 22-25 minutes, or until they are golden brown.
While the buns are still warm, brush each one with sugar-free maple syrup or regular maple syrup for added sweetness. Let cool on a wire rack for 10-15 minutes before enjoying