Preheat the oven to 300°F. Line a 9-inch square baking pan with parchment paper, letting some paper overhang on opposite sides to act as handles; set aside.
Combine the all-purpose flour, rice flour, and kosher salt in a sifter and sift into a large bowl. Whisk in 1/2 cup of the seaweed and set aside.
In a large bowl, beat the butter with an electric mixer on medium speed until fluffy, 11/2 to 2 minutes. Scrape down the sides of the bowl with a rubber spatula, add the sugar, and beat on high speed until creamy, about 2 minutes. Add the flour mixture and beat on low until the dough resembles wet sand and holds together when squeezed.
Press evenly into the prepared pan. Bake for 1 hour and 20 minutes until light golden. Set the baking pan on a wire rack and let cool for 5 minutes. Leave the oven on.
Using the parchment paper as handles, lift the shortbread out of the pan and transfer to a cutting board. Cut the shortbread (while on the paper) into thirds and then into roughly 3/4-inch-wide pieces.
Brush the tops lightly with the egg wash and sprinkle with some sugar, the remaining 2 tablespoons seaweed, and flaky sea salt. Transfer the pieces to a baking sheet and spread them out so they’re not touching.
Bake for 10 minutes, so the sides dry out a bit, and then set aside until cool.
Store in an airtight container.
Excerpted from KOREAN FOOD MADE SIMPLE © 2016 by Judy Joo. Photography © 2016 by Jean Cazals. Reproduced by permission of Houghton Mifflin Harcourt. All rights reserved.