In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3 to 5 minutes. Add the lemon zest, egg, egg yolk, and fresh lemon juice; mix until well combined.

In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, stirring gently just until incorporated. Do not overmix.

Cover the dough and refrigerate for at least 30 minutes to firm up.

Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.

Using a cookie scoop, portion the dough into balls and flatten slightly with your palm.

Bake for 10 to 12 minutes, until the edges are golden and the centers are still soft.

Let the cookies cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.

In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar, then mix in the milk and vanilla extract until smooth.

Stir in the blueberry jam until the frosting is fluffy and evenly colored.

Once the cookies are fully cool, frost them generously with the blueberry buttercream.