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 immediately and whisk in butter and lemon zest until smooth and glossy. 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 together in a mixing bowl until light and fluffy, about 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 just until dough comes together. Divide dough in half and shape each portion into a flat disk. Wrap tightly in plastic wrap and refrigerate for at least 1 hour to firm up.
03 - Preheat oven to 350°F and line baking sheets with parchment paper. On a lightly floured work surface, roll out chilled dough to 1/8-inch thickness. Using a 2-inch round cutter, cut out cookies. For half of the cookies, use a small cutter to create a window in the center. Transfer to prepared baking sheets, spacing 1 inch apart. Bake for 10-12 minutes until edges are lightly golden. Let cool on baking sheets for 5 minutes, then transfer to wire racks to cool completely.
04 - Dust the windowed cookies generously with powdered sugar using a sieve. Spread a small spoonful of chilled Meyer lemon curd onto each solid bottom cookie, leaving a small border around the edges. Gently place a windowed cookie on top and press lightly to create a sandwich. Repeat with remaining cookies. For best results, refrigerate assembled cookies for 15-20 minutes to allow the curd to set before serving.