Juice and zest your lemon if you want to use fresh lemon
In a large bowl, mix together the lemon cake mix, eggs, oil, lemon juice, and (if using) lemon zest. Stir until a thick dough forms.
Chill the dough for 20–30 minutes so it’s easier to scoop and the cookies don’t spread as much.
Preheat your oven to 350°F. Line two baking sheets with parchment paper.
Scoop the dough into 1 1/2-tablespoon portions, roll into smooth balls, and place them 2 inches apart on the baking sheets.
Bake for 8–10 minutes, until the edges are set and the centers look just barely done.
Beat the softened butter for 1–2 minutes until fluffy. Add the powdered sugar and vanilla then beat until combined (it’ll look thick at first).
Add milk or cream a little at a time and beat until smooth. For piping, keep it thicker; for spreading, add a tiny splash more.
Make sure the cookies are completely cool before frosting.
If you’re spreading the frosting, use a butter knife or small offset spatula to add a layer of vanilla buttercream on top of each cookie.
If you’re piping the frosting, scoop the buttercream into a piping bag. You can either snip the tip off the bag for a simple swirl, or use a plain round tip and pipe a swirl of frosting on top of each cookie.
Finish with a little lemon zest or sprinkles, then let the frosting set for 10–15 minutes before stacking or serving.
To store frosted cookies, let the frosting set for about 10–15 minutes first, then place them in an airtight container with parchment paper between layers so the frosting doesn’t smudge.