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 until it reaches 170-175°F. Remove from heat and whisk in butter and lemon zest until smooth and emulsified. Transfer to a bowl, cover with plastic wrap pressed directly onto the surface, and refrigerate for at least 1 hour until completely set.
02 - Cream butter and sugar together until light and fluffy, approximately 2-3 minutes. Add egg yolk and vanilla extract, mixing until fully incorporated. In a separate bowl, whisk together flour, ground almonds, cinnamon, and salt. Gradually add dry ingredients to wet ingredients, mixing until dough just comes together. Divide dough into two equal portions, flatten into disks, wrap tightly in plastic, and refrigerate for at least 1 hour.
03 - Preheat oven to 350°F and line baking sheets with parchment paper. On a lightly floured surface, roll out chilled dough to 1/8-inch thickness. Cut 2-inch rounds from the dough. For half the cookies, use a small cutter to create a decorative window in the center. Transfer to prepared baking sheets. Bake 10-12 minutes until edges are lightly golden. Let cool completely on baking sheets.
04 - Dust the windowed cookies generously with powdered sugar. Spread a rounded teaspoon of chilled Meyer lemon curd onto each solid cookie. Gently press a windowed cookie on top to create sandwiches. Refrigerate assembled cookies for 15-20 minutes to set the curd before serving.