Ingredients for the cookies:
2 sticks unsalted butter, softened
3/4 cup sugar
6 large egg yolks
1 teaspoon vanilla extract
4 ounces of almond meal
3/4 teaspoon kosher salt
1 tablespoon baking powder
1 3/4 cups all-purpose flour
Zests of 2 Meyer Lemons
Ingredients for the Meyer Lemon Buttercream:
1/2 cup sugar
2 large egg whites
1 1/2 sticks (6 ounces) unsalted butter, at room temperature
1/2 teaspoon Vanilla Extract
Zest and juice of 1 Meyer Lemon
To make the cookies:
In the bowl of a stand mixer fitted with paddle attachment, beat the butter and sugar at medium-high speed until light and fluffy. Add the egg yolks, lemon zest and vanilla extract and beat until smooth.
In a large mixing bowl, whisk the almond meal with the salt, baking powder and flour until well combined. With mixer on low, add the dry ingredients slowly to the mixing bowl and beat until ingredients are incorporated.
Pat the dough into 4 disks, wrap in plastic and refrigerate until chilled, about 1 hour.
Preheat the oven to 350° F and line 2 baking sheets with parchment paper or silpat. On a slightly floured surface, roll out the dough to 1/4-inch thick. Using a 1 3/4-inch round cutter and stamp out cookies as close together as possible. Arrange the cookies 1 inch apart on the prepared baking sheets.
Bake the cookies about 10 to 13 minutes or until lightly browned on the bottom and around the edges. Let cool on the baking sheet. Refrigerate the scraps until chilled, then reroll, stamp out more cookies and bake.
To make the Meyer Lemon Buttercream:
Put the sugar and egg whites in a large heatproof bowl over a saucepan of simmering water and whisk constantly, keeping the mixture over the heat, until it feels hot to the touch, about 3 minutes.
Pour the mixture into the bowl of a stand mixer fitted with the whisk attachment and beat the meringue on medium speed until it cools and forms a thick shiny meringue, about 5 minutes.
Switch to the paddle attachment and add the butter, one tablespoon at a time, beating until smooth. Once all the butter is in, add the vanilla extract, lemon zest and lemon juice, beat the buttercream on medium-high speed until it is thick and very smooth, 6-10 minutes. Taste for acidity, add more lemon juice if desired.
Top half of the cookies with a rounded teaspoon of the Meyer Lemon Buttercream, place another cookie on top of the buttercream and refrigerate until the buttercream is set, about 15 minutes. Keep sandwiched cookies in refrigerator. Leave at room temperature for 5 minutes before serving.