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For the cookies:
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Preheat oven to 350u00b0F. Line two large cookie sheets with parchment paper and set aside.
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Cream together butter and sugar in the bowl of a standing mixer until light and fluffy. Add lemon zest, lemon juice, vanilla extract, and eggs. Beat well. Add salt, baking powder, baking soda, and flour. Mix just until combined, scraping down the sides of the bowl to ensure you mix in all the dry ingredients.
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Measure the powdered sugar onto a plate. Scoop out a heaping teaspoon of dough and roll between your palms to create a ball about one inch in diameter. Gently roll the ball in the powdered sugar and place on the lined cookie sheet. Continue with the remaining dough, placing dough balls about three inches apart, until cookie sheets are full.
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Bake for 12 minutes and then remove pans from the oven and set them on racks. Leave the cookies on the cookie sheets for 2-3 minutes and then remove them to a cooling rack. Using the same parchment paper, continue with the remaining dough. Allow all cookies to cool completely before icing.
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For the icing:
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Lightly wipe the clean bowl of the stand mixer with vinegar to remove all traces of grease. Using a pot that does not allow the stand mixer bowl to touch the bottom, add 2-3 inches of water in the base of the pot. Then, place the mixer bowl in the pot and heat over medium heat.
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Add sugar and egg whites into the mixer bowl and simmer, stirring constantly, until temperature of the mixture reaches 140u00b0F.
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Then move the bowl to the stand mixer and use the whisk attachment. Whip at medium-high speed until mixture is thick, glossy, cool to the touch and forms stiff peaks. Switch over to the paddle attachment and beat on medium speed. Add 2-3 cubes of butter at a time until it's all incorporated, never turning off the mixer. The mixture will appear to curdle, but just keep mixing! Once icing has come to a silky, smooth texture, add vanilla and salt and mix them in.
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In a separate bowl, crush the blackberries with the back of a fork. Remove any large pits (they will be easily visible against the dark purple mixture). Add the crushed blackberries into the icing and beat until icing is purple and berries have been thoroughly mixed in.
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For the assembly:
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Once cookies have cooled completely, fill a piping back without a tip with icing. Give each cookie a generous swirl of icing. Serve immediately, or refrigerate. Once refrigerated, cookies are best served if allowed to come to room temperature, about 20 minutes.
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Adapted from Lauren's Latest.
