Preheat oven to 350°F (175°C). Lightly grease an 8-inch square baking pan and line with parchment with an overhang for easy removal. In a bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 cup almond flour, 1/4 cup granulated sugar, and 1/4 tsp fine sea salt. Add 1/2 cup cold unsalted butter cut into cubes. Using a pastry cutter or your fingertips, work the butter into the dry ingredients until mixture resembles coarse crumbs with some pea-size pieces. Add 1/2 tsp vanilla extract and 1 tablespoon ice water; toss briefly and press together just until dough holds when squeezed. If too dry, add up to 1 teaspoon more ice water. Press the dough evenly into the prepared pan, smoothing the top. Dock lightly with a fork to prevent bubbling. Bake 18–22 minutes, or until edges are light golden. Remove from oven and allow to cool 10 minutes while you prepare the curd. Reduce oven temperature to 325°F (165°C) if you will return assembled tart to oven briefly.

Lemon Curd Layer

Make lemon curd

In a medium heatproof bowl, whisk together 3 egg yolks and 1 whole egg with 1/2 cup granulated sugar until pale and slightly thickened. Add 1/2 cup fresh lemon juice and 1 tablespoon lemon zest and whisk to combine. Place the bowl over a saucepan of gently simmering water (double boiler) ensuring the bowl bottom does not touch the water. Cook, whisking constantly, until the mixture thickens and registers 170–175°F (77–80°C) on an instant-read thermometer, about 8–10 minutes. Remove from heat and strain the curd through a fine-mesh sieve into a clean bowl to remove any cooked bits and zest for a silky texture. While still warm, whisk in 4 tablespoons cold unsalted butter, a few cubes at a time, until fully incorporated and the curd is glossy. Stir in 1/8 tsp fine sea salt. Taste and adjust: if curd is too tart, whisk in 1–2 tsp granulated sugar. Let the curd cool slightly (5–10 minutes) but not set firm—pourable but thick.

Assemble and chill

Pour the warm lemon curd over the warm shortbread base and gently spread into an even layer. Refrigerate the assembled tart for at least 2 hours (preferably 3–4 hours) until the curd sets and the crust firms. If you prefer a slightly softer set, 1 hour may suffice, but chilling improves sliceability.

Blueberry Compote & Finish

Make blueberry compote

About 20–30 minutes before serving, combine 1 cup blueberries, 2 tablespoons granulated sugar, 1 tablespoon water, and 1 teaspoon fresh lemon juice in a small saucepan over medium heat. Cook, stirring occasionally, until the blueberries begin to break down and the mixture thickens, about 6–8 minutes. If you want a thicker, glossy compote, stir 1/2 teaspoon cornstarch into 1 teaspoon cold water to make a slurry and whisk it into the simmering berries; cook 1 minute more until glossy. Remove from heat and let cool to warm or room temperature.

Finish and serve

Remove the chilled tart from the pan using the parchment overhang and transfer to a serving plate. Spoon the blueberry compote over the lemon curd in a single layer or artistically as desired. Dust lightly with 1 tablespoon powdered sugar if using and garnish with fresh mint leaves. Using a sharp knife warmed in hot water and wiped dry between cuts, slice the tart into 8 square portions. Serve chilled or at cool room temperature. Store leftovers covered in the refrigerator up to 3 days; blueberry compote may be kept separate and spooned on when serving.