In a large bowl of a stand mixer with the paddle attachment, beat together butter and powdered sugar until light and fluffy; about 2 minutes.
Beat in vanilla and lemon extracts and lemon zest.
In a medium sized bowl, whisk together flour, salt and cornstarch. Stir into the butter mixture.
On a lightly floured surface, roll and shape dough into a log. (For cut-outs, for a disc about 1/4 inch thick. Wrap with plastic wrap or waxed paper and smooth. Twist the ends to close.
Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper and set aside.
Slice the cookie dough log into 1/4" discs and lay them flat on the prepared cookie sheets two inches apart.
Bake in the 350 degree oven for about 13 minutes. The cookies won't appear "done." If you're uncertain, try lifting one off the cookie sheet and checking the bottom. Cool on the cookie sheets for 20 minutes before removing to a wire rack to cool completely.
For the icing, whisk together powdered sugar and lemon juice. Lightly drizzle over cookies and top with yellow sprinkles or additional lemon zest, if desired.