Step 1

Preheat oven to 350 degrees F. Line an 8-inch-square baking pan with foil and generously coat it with cooking spray.

Step 2

To prepare crust: Combine flour, confectioners' sugar, 3 tablespoons cornstarch and 1/4 teaspoon salt in a medium bowl. Add oil and butter; using your fingertips, blend into the flour mixture until evenly combined. The mixture will be a little crumbly. Firmly press the dough into the prepared pan. Bake the crust until just barely beginning to brown around the edges, 15 to 20 minutes.

Step 3

To prepare filling: Whisk granulated sugar, cornstarch, baking powder and 1/8 teaspoon salt in a medium bowl until well combined. Whisk in eggs. Stir in water and lemon juice. Pour the filling over the crust.

Step 4

Bake until just set, 18 to 20 minutes. (The center should still be a little jiggly--it will firm up as it cools.)

Step 5

Let cool to room temperature in the pan on a wire rack, about 1 1/2 hours. Gently lift out of the pan all in one piece using the edges of the foil. Cut into 9 squares. Garnish with lemon zest and dust with confectioners' sugar, if desired, just before serving.