01 - Whisk 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, whisk in butter and lemon zest until smooth. Transfer to a bowl, press plastic wrap directly onto surface, and refrigerate at least 1 hour until set.
02 - Cream butter and sugar until light and fluffy. Add egg yolk and vanilla, mixing until fully incorporated. Whisk together flour, ground almonds, cinnamon, and salt in a separate bowl. Gradually blend dry ingredients into wet mixture until dough forms. Divide dough in half, shape into disks, wrap tightly, and refrigerate at least 1 hour.
03 - Preheat oven to 350°F. Roll chilled dough on lightly floured surface to 1/8-inch thickness. Cut 2-inch rounds. Cut small heart or circle shapes from centers of half the rounds to create windows. Transfer to parchment-lined baking sheets. Bake 10-12 minutes until edges are golden. Cool completely on baking sheets.
04 - Dust windowed cookie tops generously with powdered sugar. Spread approximately 1 teaspoon Meyer lemon curd on each solid bottom cookie. Gently press windowed cookies onto curd-topped bases. Repeat with remaining cookies. Refrigerate briefly to set filling before serving.