01 - Whisk together eggs, egg yolks, sugar, and Meyer lemon juice in a heatproof bowl. Set over a simmering pot of water to create a double boiler. Whisk constantly until mixture thickens and coats the back of a spoon, approximately 8-10 minutes, reaching 170-175°F. Remove from heat and whisk in butter and lemon zest until completely smooth. Transfer to a bowl, cover with plastic wrap pressing directly onto the curd surface, and refrigerate for at least 1 hour until set.
02 - Cream butter and sugar together until light and fluffy using an electric mixer. Beat in egg yolk and vanilla extract until fully incorporated. In a separate bowl, whisk together flour, ground almonds, cinnamon, and salt. Gradually mix dry ingredients into the butter mixture until dough forms. Divide dough in half, shape each portion into a flat disk, wrap tightly in plastic, and refrigerate for at least 1 hour.
03 - Preheat oven to 350°F. On a lightly floured surface, roll out chilled dough to 1/8-inch thickness. Cut out 2-inch diameter rounds. For half the cookies, use a small cutter to create a decorative window in the center. Transfer to parchment-lined baking sheets. Bake for 10-12 minutes until edges are golden brown. Allow cookies to cool completely on the baking sheet.
04 - Generously dust the windowed cookies with powdered sugar using a sieve. Spread approximately 1 teaspoon of chilled Meyer lemon curd onto each solid bottom cookie. Gently press a windowed cookie on top to create sandwiches. Refrigerate assembled cookies for 15-20 minutes to allow the curd to set before serving.