Spray a standard mini round cake pan with cooking spray. In the bowl of a stand mixer fitted with the paddle attachment, add the gluten-free bread flour, milk powder, sugar, instant yeast, salt and baking powder. Mix to combine.
With the mixer running on low, slowly add the water, eggs and melted butter. Increase the speed to medium and mix for 3 minutes. The dough will resemble thick cookie dough, this is normal.
Using an oiled rubber spatula, scrape down the bottom and sides of the bowl and mix again until well-combined.
Using an oiled kitchen scoop or large spoon, portion the batter evenly into the prepared mini cake pan. Use wet fingers to smooth the tops of the buns.
Cover with oiled plastic wrap and let rise in a warm place for 45-60 minutes.
In a small bowl, whisk together the egg, water and pinch of salt for the egg wash.
Using a pastry brush, lightly brush the buns with egg wash. Sprinkle with sesame seeds if desired.
Preheat the oven to 350°F. Remove the plastic wrap. Bake for 24-26 minutes, until the buns begin to turn golden brown.
Remove from the oven and turn the buns out onto a wire rack to cool completely. Slice and serve.