Shortbread Crust:

1. Preheat and Prep: Preheat oven to 350°F (177°C). Line an 8×8 inch baking dish with parchment paper, allowing it to overhang the edges.
2. Combine Dry Ingredients: In a food processor, combine flour, powdered sugar, granulated sugar, and salt. Pulse until combined.
3. Add Butter: Add cold butter pieces and pulse until coarse crumbs form.
4. Add Egg Yolks: Add egg yolks and pulse until a dough comes together.
5. Press and Bake: Press the dough into the prepared baking dish. Prick the dough with a fork. Bake for 20-25 minutes, or until lightly golden. Let cool.

Citrus Filling:

1. Reduce Oven Temperature: Reduce oven temperature to 300°F (149°C).
2. Combine Eggs and Sugar: In a saucepan over low heat, whisk together eggs, egg yolks, sugar, and flour until combined.
3. Add Citrus and Cook: Whisk in grapefruit zest, grapefruit juice, lemon zest, and lemon juice.
4. Thicken Curd: Increase heat to medium-low and whisk constantly until the curd thickens and bubbles appear on the edges (6-8 minutes).
5. Add Vanilla and Salt: Remove from heat and whisk in vanilla extract and salt.
6. Add Butter: Whisk in softened butter, one piece at a time, until fully melted and smooth.
7. Pour and Bake: Pour the citrus filling over the cooled crust. Bake for 25-30 minutes, or until the filling is set and the center is slightly jiggly.
8. Cool and Chill: Let cool completely, then refrigerate for at least 4 hours or overnight.
9. Slice and Serve: Lift the parchment paper to remove the bars. Slice into 12 or 16 pieces. Sprinkle with flaked sea salt and serve.