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1
Preheat oven to 300u00b0F.
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2
Lightly butter a 9x13-inch baking pan or quarter sheet pan.
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3
In a large bowl, cream together the butter and sugar. Add in the salt and vanilla and continue to mix until well combined. Stir in the flours until the dough is cohesive.
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4
Press the dough evenly into the prepared pan, creating a smooth surface. Using a fork, prick the dough about 20 times, keeping the marks uniform and being sure to cover all areas of the dough.
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5
Bake the shortbread for 35-40 minutes, or until golden around the edges. Remove the pan from the oven; loosen the edges of from the pan using a dull knife or offset spatula. Cool the pan for about 5 minutes.
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6
Working with the cookies while they're still warm, carefully turn the entire pan of cookies out onto a clean, flat surface. With the topside up, trim away about 1/4 inch around the border of the cookies. Crumble the scraps into a small bowl and set aside.
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7
Cut the slab of cookies into 24 squares, about 2 inches by 2 inches each.
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8
Allow squares to cool. Spread a thick layer of jam onto each cookie, and then distribute the crumbs evenly over them.
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9
Store in an airtight container in a single layer or with a piece of wax or parchment paper between layers.
