01 - Whisk together eggs, egg yolks, sugar, and Meyer lemon juice in a heatproof bowl. Set over simmering water to create a double boiler. Whisk constantly until mixture thickens and coats the back of a spoon, approximately 8–10 minutes at 170–175°F. Remove from heat and whisk in butter and lemon zest until smooth. Transfer to a bowl, cover with plastic wrap touching the surface, and refrigerate until set, at least 1 hour.
02 - Cream butter and sugar in a bowl until light and fluffy. Incorporate egg yolk and vanilla extract. In a separate bowl, whisk together flour, ground almonds, cinnamon, and salt. Gradually combine 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 dough to ⅛-inch thickness. Cut 2-inch rounds. For half the cookies, cut a small heart or circle from the center to create windows. Transfer to parchment-lined baking sheets. Bake 10–12 minutes until edges are golden. Cool completely on wire racks.
04 - Dust windowed cookies generously with powdered sugar. Spread a small amount of chilled Meyer lemon curd on each solid bottom cookie. Gently press a windowed cookie on top to create sandwiches. Repeat with remaining cookies. Refrigerate briefly to set before serving.