In a medium bowl, whisk the flour, cornstarch, baking soda, and salt together. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, sugar, and lemon zest together on medium-high speed until creamed, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg, lemon juice, and vanilla extract and beat on medium-high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl and beat again as needed to combine. The mixture will look curdled; that’s ok.

Add the dry ingredients to the wet ingredients and mix on low speed until combined. Dough will be thick and sticky. Cover and chill cookie dough in the refrigerator for at least 2 hours (and up to 3 days).

Preheat the oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. If the dough has chilled for longer than 2 hours, let it sit at room temperature for about 15 minutes.

Scoop dough, about 1.5 Tablespoons (30g) of dough each, and roll into balls. Arrange cookies 3 inches apart on the baking sheets.

Bake for 11–13 minutes or until lightly browned around the edges. The centers will look very soft and slightly underbaked. That’s key for a soft texture. Do not over-bake.

Remove from the oven and allow cookies to cool on the baking sheet for 5 minutes before transferring to a cooling rack to cool completely.

Using a food processor or blender, process the freeze-dried blueberries into a powdery crumb. Sift it through a fine mesh sieve to rid larger seeds/pieces. You should have about 2 Tbsp, or around 10g, of blueberry powder.

In a large bowl using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter and cream cheese on medium-high speed until creamy and combined, about 2 minutes. Add confectioners’ sugar, blueberry powder, lemon juice, and vanilla extract. Beat on low speed for 30 seconds, then gradually increase to high speed and beat for 2 minutes. Taste. Add a pinch of salt if frosting is too sweet.

Spread or pipe frosting on cooled cookies. I used a small offset spatula on the pictured cookies. Garnish with a lemon slice and a fresh blueberry just before serving, if desired.

Store unfrosted cookies tightly covered at room temperature for up to 1 week. Store frosted cookies tightly covered in the refrigerator for up to 5 days.