Mix the warm dairy-free milk and half of the brown sugar together. Sprinkle in the active dry yeast, give it a quick stir, and let sit for 10 minutes until frothy.
In a large bowl, mix the flour, protein powder, remaining brown sugar, salt, and cinnamon. Make a well in the center and add the yeast mixture and melted vegan butter. Mix until the dough comes together into a ball; it may be slightly sticky.
Knead the dough in a stand mixer or by hand for 8-10 minutes until no longer sticky.
Cover with a damp tea towel and let rise in a warm place until doubled in size, about 1-1.5 hours.
Roll the dough out into a long rectangle.
Prepare the filling by combining the melted Biscoff, softened butter, brown sugar, and cinnamon in a small bowl.
Spread the filling on the dough. Sprinkle the crushed Biscoff biscuits on top.
Roll the dough into a log lengthwise and cut into thick rolls.
Place the rolls in a prepared baking dish.
Cover with a damp cloth and let rise for 20-30 minutes.
Preheat the oven to 350°F while the rolls rise for a second time.
Bake for 20-22 minutes or until golden brown.
Stir together the ingredients for the frosting and ice the buns while they are still slightly warm.