Cook + cool lemon curd

1. Combine half of the butter, half of the sugar, lemon juice and lemon zest in a medium saucepan and bring to a boil.
2. Combine egg yolks with remaining half of sugar in a heat-proof bowl.
3. When the lemon juice mixture boils, slowly drizzle it into the egg yolks while whisking continuously (this is called tempering).
4. Return mixture to pot, and continue cooking on medium-low to medium heat, stirring constantly, until it boils.
5. Once large bubbles break the surface, remove the pan from the heat and whisk in remaining butter.
6. Strain the curd into a clean bowl through a mesh sieve to remove any curdled egg yolks. Add Limoncello now, if using.
7. Press plastic wrap on top, and chill.
8. Keep curd refrigerated, but serve room temperature. 

Bake macaron shells

Without Food Processor

1. In a large bowl, whisk lemon zest with almond flour and powdered sugar for a minute or so to release lemon flavor. Sift to remove large pieces of almond and lemon zest. Proceed to Step 2 below.

With Food Processor

1. If using whole/slivered almonds, grind with powdered sugar until the mixture resembles sand. Stop to scrape the bottom of the bowl at least once. Add lemon zest, and pulse four or five times. Sift dry ingredients after grinding to remove lemon zest.
2. Line two sheet pans with parchment paper or silicone baking mats.
3. Combine the egg whites and granulated sugar in the bowl of a stand mixer (hand mixer or just a whisk work, too). Whip on high speed to a stiff meringue (resembles shaving cream, smooth and glossy with stiff peaks that only slightly curve over).
4. Add the lemon juice, and whip on high speed 30 seconds more to incorporate (add food coloring here, too).
5. Add the dry ingredients and stir vigorously for a moment to combine.  Then fold the batter by scraping around the sides of the bowl and up from the bottom and over the top. Fold until it ribbons off the spatula, and becomes smooth, shiny, and flowing. 
6. Transfer to a piping bag (or large plastic bag) fitted with a small round pastry tip.
7. Pipe batter into 1.5 to 2-inch circles onto baking mats or parchment paper. If your piped shells could use a little settling out, very lightly tap the pans on the counter.  Tapping hard will cause air bubbles to pop through the surface, and all your hard work will deflate in an instant.
8. Let the piped batter rest on the counter for 20-30 minutes. The shell will turn from shiny and sticky to smooth and dull.  You should be able to gently brush your finger over the top of the shells without causing an indentation.
9. Bake @ 300° F for 15-17 minutes, until tops are hard and do not pull away from bottoms when very gently lifted. Rotate pans halfway through baking, after 8 or 10 minutes.
10. Cool completely before removing from baking mat or parchment.

Build macarons

Once the curd is cool, pipe or spoon a small amount in the middle of half the macaron shells, then top with another gently.  Store in the refrigerator for 24 hours before serving for a more intense flavor. Time spent together allows the lemony goodness of both elements bloom!