A great way to use a partial lemon is to make crispy lemon cookies. Plan for time for butter to soften before using. Also allow for dough chill time to help with the consistency. These are delicious and easy to make!
Set butter out to get to room temperature 30 minutes in advance.
In a large mixing bowl, cream together butter and sugar with mixer or by hand until light and fluffy.
Add in the lemon juice, lemon zest, and pure vanilla extract. Mix and set aside.
In medium bowl, combine flour, cornstarch, baking soda, and salt (if using).
Slowly add the flour mixture into the wet ingredients. Mix until incorporated but do not overmix. The dough should be soft.
Leave dough in the bowl and seal completely with plastic wrap. Set in refrigerator for 30 - 60 minutes or the freezer for 15 - 30 minutes.
Midway through the dough chilling time, preheat oven to 325°F (160°C). Line cookie sheet with parchment paper.
Scoop dough into 1-inch balls then round them in your hands. Place 12 dough balls (if using standard-sized baking sheet) onto the prepared tray.
Using the flat bottom of a glass, gently press down into each dough ball. If the dough sticks to the glass, dust it with flour. For thin cookies, they should be about ¼ inch thick before baked.
Place in oven on center rack, and bake for 10 - 12 minutes. Oven times vary. Turn on the oven light; remove cookies when the edges turn lightly browned.
While the first batch is baking, scoop out the remaining dough balls. If you have a second cookie tray, line it, and set the dough balls on the tray until the first batch is done baking. Otherwise, leave them in the bowl where the dough was.
Remove baking sheet from oven, and set to cool. Leave cookies on the sheet. After 3 minutes, remove them onto a cooling rack.
Set the second batch of cookies in the oven to bake and repeat steps 10 and 12. Repeat for the remaining dough.
Notes
Be sure butter is at room temperature.
Don't overmix the dough. Combine ingredients but don't overmix.
Chill the dough before baking to prevent the cookies from spreading. While they are meant to be thin, chilling helps to solidify the butter.
Flatten dough after forming dough balls so they are thin after baking.
Store in airtight container on counter or in freezer, not in refrigerator.