Prepare the Crust:
In a small bowl, whisk together the buttermilk, egg yolks, and vanilla. Set aside.
In a food processor, pulse the flour, powdered sugar, and salt to combine.
Add the cold butter and pulse until the mixture resembles pea-sized crumbs.
While the processor is running, pour in the wet ingredients and mix until the dough starts coming together.
Transfer the dough onto plastic wrap, form into a disc, and wrap tightly. Refrigerate for 1 hour or freeze for 15-20 minutes.
Preheat oven to 375°F (190°C).
Press the chilled dough into a greased 8×8″ pan, covering the bottom and slightly up the sides.
Wrap the pan in plastic wrap and freeze for 20 minutes.
Remove the wrap and bake for 20-25 minutes until set but not golden.
Lower oven temperature to 325°F (163°C).

Prepare the Filling:
In a small bowl, gently mix the raspberries with 1/4 cup sugar. Let sit while making the filling.
In a separate bowl, whisk together the eggs, remaining 1 ½ cups sugar, flour, and lemon zest until pale yellow.
Stir in the lemon juice until combined.

Assemble & Bake:
Scatter the raspberries over the warm crust.
Pour any juices from the raspberries into the lemon filling, whisk again, then pour over the crust.
Bake at 325°F for 20-30 minutes, until the center is set and no longer jiggles.
Let cool completely before slicing.
Serve immediately or wrap and refrigerate overnight for best flavor.