Step 1

Place a rack in middle of oven and preheat to 350°. Line an 8x8" baking pan, preferably metal, with overlapping sheets of parchment paper, leaving generous overhang on all sides. Whisk flour, granulated sugar, salt, and baking powder in a large bowl to combine. Add butter and vanilla and mix with a rubber spatula until butter is evenly distributed. The dough should stick together in clumps while still remaining crumbly.

Step 2

Transfer dough to prepared pan (you may find it easier to form into a ball first instead of working with clumps—entirely up to you) and press into pan in an even layer, working it all the way to the sides and corners.

Step 3

Bake shortbread until top and edges are golden brown, 13–17 minutes. Transfer to a wire rack and let cool.

Step 4

While the shortbread is cooling, whisk flour, cream of tartar, and salt in a small bowl to combine. Vigorously whisk eggs, egg yolks, passion fruit juice, lemon juice, and granulated sugar in a large saucepan just to combine. Add dry ingredients and whisk to incorporate.

Step 5

Set saucepan over medium heat and cook curd, whisking constantly and making sure to get into the corners of the pan, until it begins to release steam and thicken, 9–12 minutes. Immediately remove pan from heat, add butter, and whisk until smooth.

Step 6

Pour hot curd through a fine-mesh sieve onto shortbread crust, using a rubber spatula to scrape off any curd that clings to the bottom of sieve. Spread curd into an even layer, working it all the way to the sides and corners. Tent a sheet of foil over baking dish to cover loosely and bake 12 minutes.

Step 7

Reduce oven temperature to 325° and remove foil from pan. Continue to bake bar until sides of curd are set but center still jiggles slightly when pan is gently shaken, 8–10 minutes more. Transfer pan to a wire rack and let bar cool, about 1 hour. Cover pan tightly with plastic wrap and chill at least 3 hours before serving.

Step 8

Just before serving, slice passion fruit bars into 2" squares and top with spoonfuls of fresh passion fruit or dust with powdered sugar with a fine-mesh sieve if desired.Do ahead: Bars can be baked 3 days ahead. Keep chilled.