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1
Preheat the oven to 300.
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2
Grease a 9-inch-square baking pan and line with parchment paper, leaving a few inches of overhang at 2 ends.
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3
Line a rimmed baking sheet with parchment paper.
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4
In a medium bowl, whisk the 2 flours with the kosher salt.
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5
Place the seaweed in a small resealable plastic bag and lightly crush with a rolling pin.
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6
Whisk 1/2 cup of the crushed seaweed into the dry ingredients.
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7
In a large bowl, using a hand mixer, beat the butter at moderately high speed until light and fluffy, about 2 minutes.
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8
Add the 1/2 cup of sugar and beat until the mixture is smooth and pale yellow, scraping down the side of the bowl as needed, about 2 minutes more.
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9
Beat in the dry ingredients; the mixture will resemble wet sand and hold together when pressed.
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10
Transfer the dough to the prepared pan and pat into an even layer.
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11
Bake for about 1 hour and 20 minutes, or until the shortbread is a light golden brown.
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12
Let cool on a rack for 5 minutes.
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13
Using the parchment overhang, transfer the shortbread to a work surface.
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14
Brush the top with the egg wash and sprinkle with sugar, flaky sea salt and the remaining 3 tablespoons of crushed seaweed.
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15
Cut the shortbread square into thirds, then slice crosswise into 3/4-inch-wide cookies.
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16
Transfer to the prepared baking sheet and bake until golden brown on top, about 10 minutes.
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17
Transfer the cookies to a rack and let cool completely before serving.
