Step 1 Heat the oven to 350 degrees. In a medium bowl, combine the melted butter with 1 cup flour and the powdered sugar, stirring just until the dough comes together. Pat the dough into an 8-inch square pan and bake just until lightly golden, about 18 minutes. Remove and cool the pan on a wire rack to room temperature.
Step 2 In a medium bowl, whisk together the granulated sugar, baking powder and remaining 2 tablespoons flour. Whisk in the lemon juice and eggs until thoroughly combined. Pour the custard into the pan over the baked crust and bake again until it's set, 20 to 25 minutes. Remove and cool the tart on a wire rack.
Step 3When cooled, dust lightly with additional powdered sugar and cut into bars. The lemon bars will keep for 1 week, refrigerated.