Preheat oven to 350°F (175°C). Line an 8x8 pan with parchment paper, leaving overhang on two sides.

In a medium bowl, mix together the flour, sugar, salt, and poppy seeds.

Add the softened butter in pieces, using your fingers to pinch and rub it into the flour mixture until no dry spots remain.

Transfer the mixture to the prepared baking pan and press into an even layer across the bottom.

Bake for 13–15 minutes, until the edges are lightly golden and the center is set but pale.

Reduce oven to 325°F (165°C).

In a small saucepan, cook the blueberries, zest, and lemon juice over medium-low heat until the berries are fully broken down, about 10 minutes.

Strain through a fine mesh sieve set over a bowl, pressing well to extract as much liquid as possible.

Cool for 5–10 minutes.

In a separate bowl, whisk together the sugar, cornstarch, and salt. Add the eggs and yolks, whisking until smooth.

Whisk in the cooled blueberry mixture and return everything to the saucepan over medium-low heat.

Cook, whisking continuously until the mixture is thick enough to coat the back of a spoon, about 5–8 minutes.

Turn off the heat and stir in the butter and vanilla until fully melted.

Strain the curd through a fine mesh sieve into a bowl.

Pour into the crust, smoothing to the corners with an offset spatula.

Bake at 325°F for 13–15 minutes, until the edges are set and the center jiggles slowly.

Cool at room temperature for 15-20 minutes.

In a small saucepan, whisk together the sugar, salt, and cornstarch.

Add the eggs and egg yolk and mix until smooth. Stir in the lemon zest and juice.

Place over medium-low heat, whisking continuously until thickened, about 5–7 minutes.

Turn off the heat and stir in the butter until fully melted.

Strain through a fine mesh sieve into a bowl.

Pour gently over the blueberry layer, smoothing evenly to the edges.

Bake at 325°F for 12–14 minutes, until edges are set and the center slightly jiggles.

Cool to room temperature, then refrigerate for at least 4 hours, or preferably overnight.

Lift out using the parchment overhang. Slice with a hot, clean knife into 9 large or 16 small squares.

Dust with powdered sugar, optionally mixed with freeze-dried blueberry powder.