Start by mixing flour, powdered sugar, salt, and cold butter until crumbly. Press this mixture firmly into a lined baking pan to create an even base. Bake until lightly golden, which helps form a sturdy and flavorful crust.
Cook fresh blueberries in a saucepan until they break down and release their juices. Stir occasionally to prevent burning and ensure even cooking. Once softened, blend or mash them into a smooth puree for the curd.
In a bowl, whisk together sugar, eggs, egg yolks, lemon juice, and cornstarch. Add the blueberry puree and mix until fully combined. This mixture will transform into a thick, creamy curd once heated.
Transfer the mixture to a saucepan and cook over medium heat, stirring constantly. It will gradually thicken into a smooth, glossy curd. Remove from heat and stir in butter and vanilla for extra richness.
Pour the prepared blueberry curd over the pre-baked crust. Spread it evenly to cover the entire surface. This layer will set beautifully as it bakes again.
Bake until the curd is set but still slightly jiggly in the center. Allow the bars to cool completely, then refrigerate for a few hours. Chilling helps the bars firm up and enhances their flavor.