Make the lemon curd filling first since it will need 2-3 hours to chill before assembling the cookies.

Place a strainer over a medium-sized bowl near the stove.

Place the sugar in a small bowl and zest the lemons onto the sugar. Use your fingers to rub the sugar and zest together. Juice the lemons until you have 6 tablespoons of juice.

In a non-reactive 2 quart saucepan, combine the whole egg, egg yolks, and lemon-sugar and whisk until well blended. Stir in the lemon juice, butter, and salt. The butter will be in solid pieces.

Cook the mixture over medium heat, whisking constantly until the mixture is thickened. As you cook the mixture the butter will melt. When the mixture has thickened and reached a temperature of 185°F pour it into the strainer. Press the curd through the strainer with a spatula to remove the solids.

Cover the surface of the curd with plastic wrap, gently pressing the plastic wrap onto the surface of the curd. Place the curd in the refrigerator and chill.

The lemon curd is good in the refrigerator for 2 weeks and can be frozen for up to 1 year. If freezing the curd package it in a freezer container after it has chilled. When ready to use place it in the refrigerator to thaw for 24 hours before intended use.

Place the sugar in the bowl of an electric stand mixer. Zest the lemons directly onto the sugar, until there is about three (3) teaspoons of zest.

Add the butter and salt to the lemon sugar mixture. Mix on medium speed with the paddle attachment until the mixture is smooth and looks creamy about 2 minutes.

Stop the mixer, scrape down the bowl, mix in the egg yolk, then mix in the lemon juice. Scrape the bottom of the bowl. Add all the flour at once, then mix it in on low speed until all the flour is well incorporated. The dough will be crumbly at this point. Increase the mixer speed to medium and mix until the dough has completely come together and formed large clumps of dough.

Turn the cookie dough out onto a lightly floured surface. Use your hands to shape the dough into a rectangle and lightly flour the top of the dough. Do not use too much flour, just enough to keep the rolling pin from sticking.

Chill the dough in the refrigerator for at least 20 minutes. Chilling the dough will help the cookies hold their shape when baking.

Take the lemon curd out of the refrigerator and stir it to loosen it up. Lay out your cookies. Pair a bottom cookie with a top cookie.

Place about 1 teaspoon of lemon curd in the center of the bottom cookie, then lightly press the top cookie on top of the curd until it spreads to the edges and up through the opening.

If serving the assembled cookies immediately they can sit at room temperature for up to 2 hours. If not serving the cookies immediately, refrigerate them in an airtight container for up to 3 days. The longer the cookies sit, the softer they will become because of the lemon curd.