In a medium bowl, soften the butter with a spatula. Add the brown sugar and salt, pressing the sugar into the butter until fully combined.
Switch to a hand mixer on high speed. Whip for 2–3 minutes until the mixture turns pale, light, and fluffy.
Add half of the room-temperature egg whites. Whip on high for 1–2 minutes, then add the remaining egg whites and repeat whipping until the mixture is completely smooth and emulsified.
Add the fine rice flour all at once. Use a spatula to fold gently until no traces of dry flour remain and a smooth dough forms.
Transfer the dough into a piping bag fitted with star piping tubes or any piping tips you prefer.
On a parchment-lined baking sheet, pipe your designs. To create the puppy look, pipe two small rounds for the head/muzzle, and use a star tip to lift and pull away for a textured "fur" look on the ears.
Bake at 160°C (320°F) for 15 minutes. Rotate the baking tray for an even bake, and bake for 2 more minutes until they reach a perfect kitsune-iro (golden-brown) edge. Allow to cool completely on a wire rack.
Melt a tiny amount of dark chocolate. Use a toothpick or a fine piping cone to dot on the eyes and nose to bring your puppy cookies to life.