I love classic shortbread just as much anyone; but I also love to make shortbread with ground nut flours to take advantage of the flavor of the nuts and make the cookies gluten-free.
Almond flour — the most common nut flour in grocery stores today — is a great canvas for flavor: Here I use clementine or orange zest for brightness and a poppy seed jam filling for a pleasantly bitter tang. Since the proper amount of almond flour is crucial to getting the texture of the cookie just right, I call to pack the flour in the cup like brown sugar, so each cup is the same; if you have a scale, I’ve provided the weight, as well.
Half the dough gets pressed into a pan and par-baked then spread with poppy seed jam and covered in the rest of the dough, which is grated on with a box grater. I learned this technique from a Hungarian shortbread recipe, and it produces small, super-crisp nubbly bits of shortbread on top, good for both variance of texture and to better hold all the blue, silver and white sugar dragées.
Heat the oven to 350 degrees. Grease a 9-by-13-inch metal baking pan, line the bottom and two long sides with a sheet of parchment paper and grease the paper as well.
In a large bowl, combine the butter, sugar, baking powder, salt, vanilla, zest and egg yolks and beat on medium speed of a mixer until fluffy, about 3 minutes (see Baker’s Note). Add the almond flour and beat on low speed until just combined.
Scoop half the dough into the prepared baking pan and use your hands to press it evenly over the bottom. Place the baking pan in the oven and bake until light golden brown at the edges, about 15 minutes. Scoop the other half of the dough into a ball, wrap in plastic and refrigerate while the base bakes and you make the filling, at least 1 hour. Once the base is ready, transfer the baking pan to a rack and let cool to room temperature.
While the cookie base bakes, make poppy seed jam: In a small saucepan, combine the sugar, poppy seeds, juice, butter, salt and lemon zest and bring to a simmer over medium heat. Cook, stirring often, until thickened like bubbling lava, 4 to 5 minutes. Remove the pan from the heat, stir in the vanilla and let the jam cool to room temperature.
Once the ball of dough has chilled, remove it from the refrigerator. Spread the jam onto the par-baked cookie base, leaving a half-inch border on each edge. Gently grate the chilled dough on the large holes of a box grater while holding the grater directly over the poppy seed jam. Gently spread the grated dough into an even layer. Sprinkle the dough with blue, silver, and white sugars.
Bake until the top is golden brown in spots on top and the cookie base is fully set, about 30 minutes. Remove the pan from the oven and let the bars cool completely in the pan.
Use the overhanging parchment paper to lift the bars out of the pan and transfer to a cutting board. Cut into 24 squares and sprinkle with more colored sugars, if you like.
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