01 - Whisk together eggs, egg yolks, sugar, and Meyer lemon juice in a heatproof bowl. Set over a simmering pot of water, whisking constantly until mixture thickens and coats the back of a spoon (8-10 minutes, reaching 170-175°F). Remove from heat and whisk in butter and lemon zest until smooth. Transfer to a bowl, cover with plastic wrap directly touching the surface, and refrigerate for at least 1 hour until set.
02 - Cream butter and sugar until light and fluffy. Add egg yolk and vanilla, mixing until fully incorporated. In a separate bowl, whisk together flour, ground almonds, cinnamon, and salt. Gradually fold dry ingredients into wet mixture until dough forms. Divide dough in half, shape into two disks, wrap separately, and refrigerate for at least 1 hour.
03 - Preheat oven to 350°F. Roll out dough on a lightly floured surface to 1/8-inch thickness. Cut 2-inch rounds from the dough. For half the rounds, cut a small heart or circle from the center to create windows. Transfer to parchment-lined baking sheets. Bake for 10-12 minutes until edges are golden brown. Cool completely on baking sheets.
04 - Generously dust the windowed cookies with powdered sugar. Spread a small spoonful of chilled Meyer lemon curd on each solid bottom cookie. Gently press a windowed cookie on top to form sandwiches. Repeat with remaining cookies. Refrigerate briefly to set before serving.