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1
Cream the butter in a mixer fitted with a paddle attachment (or using a hand mixer) until soft and fluffy.
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2
Add the egg yolks and vanilla and mix well.
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3
Mix the granulated sugar, flour, baking powder, and salt together.
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4
Add to the butter and egg yolk mixture and mix just until incorporated and the dough starts to come together.
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5
Turn the dough out onto a floured work surface and form into two balls.
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6
Wrap each ball in plastic wrap and freeze at least 2 hours or overnight (or as long as a month, if you like).
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7
Heat the oven to 350 degrees.
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8
Lightly grease a 8x8 or 9x9 square baking pan or a 10-inch tart pan with removable bottom.
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9
(I use Baker's Joy spray).
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10
Remove one ball of dough from the freezer and coarsely grate it by hand or with the grating disk in a food processor into the bottom of the greased baking pan (or tart pan).
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11
Make sure the surface is covered evenly with shreds of dough.
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12
Gently mix the blueberries, raspberries and the lemon juice in a medium bowl.
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13
Mix sugar, flour, lemon zest in a small bowl until combined, then add to blueberry/raspberry mixture and mix gently with your hand to coat the berries with the flour mixture.
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14
Cover the bottom layer of dough with the blueberry mixture as evenly as possible, gently pressing the berries into the dough.
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15
Remove the remaining dough from the freezer and coarsely grate it over the entire surface.
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16
(I grate this second ball of dough onto a piece of plastic wrap, then sprinkle over the berries.
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17
).
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18
Place in oven and cover lightly with foil, bake for 20 minutes, then remove foil.
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19
Continue baking until lightly golden brown and the center no longer wiggles, about 20-30 minutes longer.
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20
Remove from oven and allow to cool.
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21
After the shortbread is cooled, sprinkle with powdered sugar, if desired or drizzle with lemon glaze.
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22
For lemon glaze, mix 1/2 cup powdered sugar with 1 tablespoon lemon juice.
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23
Add enough milk, 1 teaspoon at a time, to thin out to thick drizzling consistency.
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24
Cool on a wire rack, then cut in the pan with a serrated knife.
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25
Chilling the pan in the fridge makes it a lot easier to get clean cuts.
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26
Adapted from Butter Sugar Flour Eggs.
