1. Preheat oven to 350 degrees. Line an 8x8-inch pan with parchment, leaving some overhang.

To make the crust:

1. In a medium bowl, whisk together flour, powdered sugar, brown sugar, and salt.
2. Add cold cubed butter and vanilla (if using). Cut in with a pastry cutter or your fingers until mixture resembles coarse crumbs.
3. Press firmly and evenly into the prepared pan.
4. Bake for 18–20 minutes, or until edges are lightly golden.
5. In a medium bowl, whisk together sugar, flour, baking powder, and salt.
6. Add eggs, lemon juice, and zest. Whisk until smooth but don’t overbeat (prevents too much air).
7. Pour over hot crust and bake again for 20–23 minutes, or until center is set and no longer jiggles when shaken. It should not look wet in the middle.
8. Cool completely in the pan on a wire rack. Chill in the fridge for at least 1 hour for clean slices.
9. Dust with powdered sugar before serving.