Move your oven rack to the middle position and preheat to 450 degrees F (230 C).
Use a blender or spice grinder to the the katsuobushi and rolled oats into a fine powder.
Add the powdered katsuobushi, oats, all-purpose flour, potato starch, powdered sugar and salt to the bowl of a food processor and pulse a few times to mix.
Add the butter and pulse for 2 seconds at a time until the mixture starts to come together in big clumps.
Dump the dough onto a non-stick surface such as a sheet of parchment paper, or silicone mat. Working quickly, shape the dough into a 1/2-inch thick square (7-inch x 7-incby wrapping the edges of the parchment paper over the dough and pressing the dough into the corners.
Refine the edges of the dough using a pastry knife and as a guide to make a perfect square that's roughly the same thickness. Put the dough in the freezer for 10 minutes.
Cut the square of dough in half and the cut the dough into 20 bars. Place them on a baking sheet. Use the blunt end of a toothpick to poke a few holes into each piece of shortbread.
Place the sheet pan in the oven and bake for 3 minutes.
Turn down the heat to 250 degrees F (120 C) and continue baking until the shortbread is a pale golden brown (8-12 minutes). Turn the oven off, and prop the oven door open a crack with a wooden spoon let the shortbread dry out for another 45 minutes.
Remove the shortbread from the oven and transfer to a cooling rack to cool completely.
Melt the chocolate either in a double boiler or in the microwave. And prepare a sheet lined with parchment paper or a silicone mat.
Dip just the foot of each shortbread in the chocolate, let the excess drip off, and then gently place each cookie on the prepared sheet. Let the chocolate solidify in a cool place.