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 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 in a bowl 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 mix dry ingredients into wet mixture until dough forms. Divide dough in half, shape into disks, wrap, and refrigerate for at least 1 hour.
03 - Preheat oven to 350°F. On a lightly floured surface, roll out dough to 1/8-inch thickness. Cut out 2-inch diameter rounds. For half the cookies, 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. Cool completely on wire racks.
04 - Dust windowed cookies generously with powdered sugar. Spread a small spoonful of chilled Meyer lemon curd on each solid bottom cookie. Top with a windowed cookie, pressing gently to form sandwiches. Repeat with remaining cookies. Refrigerate briefly to set before serving.