In a large mixing bowl, combine 2 cups of flour, granulated sugar, instant yeast, baking powder, and salt. Whisk thoroughly until the dry ingredients are well blended and free of lumps.
In a separate bowl, blend the warmed milk and melted butter. Once combined, whisk in the eggs and vanilla extract until smooth.
Gradually incorporate the wet mixture into the flour mixture, stirring gently with a wooden spoon or spatula until a shaggy dough begins to form.
Slowly add the remaining flour, 1/2 cup at a time, kneading the dough by hand or with a dough hook. Continue kneading for 8-10 minutes, or until the dough is smooth, elastic, and no longer sticky.
Lightly grease a large bowl and place the dough inside. Cover it with a damp kitchen towel and allow it to rise in a warm area until it has doubled in size, approximately 1 to 1.5 hours.
Once the dough has risen, gently punch it down to release air and transfer it to a floured surface. Roll the dough into a rectangle measuring about 16x12 inches.
Evenly spread the softened butter for the filling across the rolled-out dough. In a small bowl, combine the brown sugar, ground cinnamon, and nutmeg (if using), and then sprinkle this mixture generously over the buttered dough.
Starting from one of the longer edges, carefully roll the dough into a tight log, ensuring the filling remains inside. Pinch the seam to seal the log thoroughly.
Cut the rolled dough into 12 equal slices using a sharp knife or a bench scraper. Place the sliced rolls in a greased baking dish, spacing them evenly. Cover with a towel and allow them to rise again for 30 minutes, until puffed.
Preheat your oven to 350°F (175°C). Once ready, bake the rolls for 25-30 minutes or until they have turned a lovely golden brown on top.
While the rolls are baking, prepare the icing by combining the icing sugar and milk in a small bowl. Mix until you achieve a smooth and pourable consistency, adjusting as necessary by adding more icing sugar for thickness or milk for thinning.
Once the rolls are out of the oven, let them cool for a few minutes. While they are still warm, drizzle the icing generously over the top, allowing it to melt slightly and soak in.