Preheat the oven to 250°F.
Put the butter, brown sugar, and salt in the bowl of an electric mixer with a paddle attachment and beat on medium speed for 10 minutes.
Add the sifted pastry and bread flours, and mix until combined. Scrape down the bowl and the paddle, and mix again briefly.
Roll out on a well-floured board until
Put the shortbread on cookie sheets and bake in the oven until caramelized, about 2 hours.
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