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 clean bowl, pressing plastic wrap directly onto the surface to prevent skin formation, and refrigerate for at least 1 hour until fully set.
02 - Cream butter and granulated sugar in a mixing bowl until light and fluffy, approximately 2–3 minutes. Beat in egg yolk and vanilla extract 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 into two equal portions, shape into flat disks, wrap tightly in plastic, 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 chilled dough to ⅛-inch thickness. Using a 2-inch round cutter, cut out cookies. For half of the rounds, use a small heart or circle cutter to remove the center, creating windowed tops. Transfer to prepared baking sheets, spacing 1 inch apart. Bake for 10–12 minutes until edges are lightly golden. Cool on pans for 5 minutes, then transfer to wire rack to cool completely.
04 - Generously dust windowed cookies with powdered sugar using a fine-mesh sieve. Spread approximately 1 teaspoon of chilled Meyer lemon curd onto each solid bottom cookie, leaving a small border around the edge. Gently press a windowed cookie onto each filled cookie to create sandwiches. Refrigerate assembled cookies for 15–20 minutes to allow filling to set before serving.