Preheat oven to 350°F (175°C)
In a medium bowl, sift together the flour, cornstarch, baking soda, and salt
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy
Add the lemon zest and mix to fully incorporate
Beat in the egg and lemon juice
Gradually add the dry ingredients to the butter mixture until fully combined
Fold in the vanilla extract
Chill the dough in the refrigerator for at least 1 hour
Roll the dough into 1-inch balls and place on a parchment-lined baking sheet, spacing them evenly
Bake for 10–12 minutes, until the edges are lightly golden
While the cookies cool, prepare the blueberry frosting: beat the cream cheese until smooth, then gradually add confectioners’ sugar and a splash of fresh lemon juice until creamy
Once the cookies are firm, spread or pipe the frosting over each cookie
Let the frosting set before serving or storing