01 - Whisk eggs, egg yolks, sugar, and Meyer lemon juice in a heatproof bowl. Set over a pot of simmering water to create a double boiler. Whisk continuously until mixture thickens enough to coat the back of a spoon, approximately 8 to 10 minutes, reaching 170°F to 175°F. Remove from heat and whisk in butter and lemon zest until smooth and emulsified. Press plastic wrap directly onto surface and refrigerate until set, at least 1 hour.
02 - Cream butter and sugar together until pale and fluffy. Beat in egg yolk and vanilla extract until fully incorporated. In separate bowl, combine flour, ground almonds, cinnamon, and salt. Gradually mix dry ingredients into butter mixture until cohesive dough forms. Divide dough in half, shape each into a disk, wrap tightly, and refrigerate for at least 1 hour.
03 - Preheat oven to 350°F. On lightly floured surface, roll chilled dough to 1/8-inch thickness. Using a 2-inch round cutter, cut circles from dough. For half of the rounds, use a small heart or round cutter to create windows in centers. Transfer to parchment-lined baking sheets, spacing 1 inch apart.
04 - Bake cookies for 10 to 12 minutes until edges turn golden brown. Centers should remain pale. Let cool on baking sheets for 2 minutes before transferring to wire rack to cool completely.
05 - Generously dust windowed cookie tops with powdered sugar. Spread 1/2 teaspoon chilled Meyer lemon curd onto each solid bottom cookie. Gently press a windowed cookie on top to create sandwich. Repeat with remaining cookies. Refrigerate assembled cookies for 15 minutes to set curd before serving.