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, about 8-10 minutes, reaching 170-175°F. Remove from heat and whisk in butter and lemon zest until smooth. Transfer to a bowl and cover with plastic wrap directly touching the surface. Refrigerate for at least 1 hour until set.
02 - Cream butter and sugar together until light and fluffy using an electric mixer. Beat in egg yolk and vanilla extract until fully incorporated. In a separate bowl, whisk together flour, ground almonds, cinnamon, and salt. Gradually mix the dry ingredients into the wet ingredients until dough forms. Divide dough in half, shape into two disks, wrap in plastic, 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 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 brown. Let cool completely on baking sheets.
04 - Dust the windowed cookies generously with powdered sugar. Spread about 1 teaspoon 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 the curd before serving.