2 medium lemons grated for 2 teaspoons of zest, and squeezed for 1/4 cup fresh lemon juice
2 tablespoons salted butter, softened
2 tablespoons confectioners' sugar
Optional raspberry sauce:
1 pound frozen raspberries
1/4 cup sugar
2 teaspoons lemon juice
1 tablespoon water, more or less as needed
Preheat oven to 325°F (163°C)
Prepare the crust: Cream butter and sugar in medium bowl with electric or stand mixer on high speed. Add vanilla and mix until combined. Add flour and hazelnuts and mix at low speed until fully incorporated. Press dough evenly into bottom of an 8-by-8-inch baking pan. Refrigerate until firm, approx 30 min. Prick crust with fork and bake for 30 minutes or until crust turns golden brown. Cool on rack to room temperature.
Prepare the lemon curd: Blend the egg yolks with the cornstarch and sugar in a medium non-aluminum pan. Place over low heat and slowly whisk in water and lemon juice. Increase heat to medium-low and cook, stirring constantly, until mixture thickens enough to coat back of spoon. Remove from heat. Add lemon zest and butter and cool for 10 minutes.
Assemble the bars: Spread lemon curd evenly over crust with spatula. Place in centre of oven and bake 30 to 40 minutes or until edges begin to turn light golden brown. Cool to room temperature on rack, then chill in refrigerator for at least 1 hr before cutting into bars. Dust top with confectioners' sugar.
Prepare optional raspberry sauce: Combine raspberries, sugar, water, and lemon juice in a saucepan over medium heat. Cook and stir until raspberries break down, sugar dissolves, and sauce is heated through until it thickens slightly. Remove from heat and press sauce through a fine-mesh strainer to remove seeds. Cool to room temperature, cover the bowl with plastic wrap, and refrigerate until chilled.