01 - Whisk together eggs, egg yolks, sugar, and Meyer lemon juice in a heatproof bowl. Set over simmering water creating a double boiler. Whisk constantly until mixture thickens enough to coat the back of a spoon, reaching 170-175°F, approximately 8-10 minutes. Remove from heat, whisk in butter cubes and lemon zest until fully incorporated and smooth. Press plastic wrap directly onto curd surface and refrigerate for at least 1 hour until set.
02 - Cream butter and sugar together until light and fluffy. Beat in egg yolk and vanilla extract until fully combined. In separate bowl, whisk together flour, ground almonds, cinnamon, and salt. Gradually incorporate dry ingredients into wet mixture until dough forms. Divide dough into two equal portions, flatten into disks, wrap tightly, and refrigerate for minimum 1 hour.
03 - Preheat oven to 350°F. Roll chilled dough on lightly floured surface to 1/8-inch thickness. Cut 2-inch rounds from dough. Using small cutter, remove centers from half the rounds to create windows. Arrange cookies on parchment-lined baking sheets. Bake 10-12 minutes until edges turn golden brown. Transfer to wire rack and cool completely.
04 - Dust windowed cookie tops generously with powdered sugar. Spread approximately 1 teaspoon Meyer lemon curd onto each solid bottom cookie. Gently press windowed cookie atop curd-filled base to form sandwich. Repeat with remaining cookies. Refrigerate assembled cookies 15-20 minutes to set filling before serving.