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 immediately. Whisk in butter and lemon zest until smooth and fully incorporated. Transfer to a clean bowl and press plastic wrap directly onto the surface. Refrigerate for at least 1 hour until completely set.
02 - Cream butter and sugar in a mixing bowl 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 mixture, mixing until dough just comes together. Divide dough into two equal portions. Shape each into a flat disk, wrap tightly in plastic, and refrigerate for at least 1 hour until firm.
03 - Preheat oven to 350°F. Line baking sheets with parchment paper. On a lightly floured surface, roll out dough to 1/8-inch thickness. Cut out 2-inch rounds. For half the cookies, use a small cutter to create a decorative window in the center. Transfer to prepared baking sheets, leaving 1 inch between cookies. Bake for 10-12 minutes until edges are golden brown. Cool completely on baking sheets.
04 - Generously dust windowed cookies with powdered sugar using a sieve. Spread approximately 1 teaspoon of chilled Meyer lemon curd on each solid bottom cookie, leaving a small border. Gently press a windowed cookie on top to create sandwiches. Repeat with remaining cookies. Refrigerate assembled cookies for 15-20 minutes to allow filling to set before serving.