In a medium saucepan, bring raspberries, sugar, lemon zest, and lemon juice to a boil over medium heat. Reduce heat to medium-low, and simmer for 5 minutes. Transfer mixture to the container of a blender, and process until smooth. Strain mixture through a fine-mesh sieve; discard solids. Return mixture to saucepan, and heat over medium heat.
In a medium bowl, whisk together egg yolks, cornstarch, and salt. Add about ¼ cup hot raspberry mixture to eggs, whisking constantly. Add egg mixture to remaining hot raspberry mixture in pan. Cook over medium-high heat, whisking constantly, until thickened, 5 to 7 minutes. Remove from heat, and add butter, 1 tablespoon at a time, whisking to combine after each addition.
Let cool completely in refrigerator before using, at least 2 hours. (Curd will keep refrigerated for up to 1 week.)
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/macarons/