Place an oven rack in center of oven. Preheat oven to 350 degrees F. Line an 8 x 8 x 2-inch baking pan with foil. Grease foil; set aside.
For the crust, in a food processor combine the flour, spices, salt, and sugars. Pulse just until mixed. Add cold butter; pulse several times to break up pieces. Process about 1 minute, just until dough forms moist popcornlike clumps that hold together when pressed between fingers. Turn dough into prepared pan, pressing evenly. Bake crust for 17 to 20 minutes, until firms and bubbly. Cool on wire rack.
Meanwhile, for custard, in a mixing bowl beat the sugar, flour, baking powder, orange juice, and lemon juice with electric mixer for 1 minute, scraping down side of bowl as needed. Remove bowl from turntable. With a spatula, swirl custard a couple of times in bowl; lightly tap bowl against counter to remove any air bubbles. Pour custard into crust, tiling pan to evenly smooth the thin layer.
Bake the custard-topped crust about 20 minutes, until golden and set. Cool completely on a wire rack. Dust with sugar. Cut in 2-inch squares. Store in refrigerator, or wrap and freeze.