Preheat the oven to 350°F. Grease an 8x8-inch baking pan and line it with parchment paper, leaving some overhang for easy removal.
In a medium bowl, combine the flour and powdered sugar. Cut in the softened butter until the mixture resembles coarse crumbs. Press this mixture evenly into the bottom of the prepared baking pan. Bake for 15 minutes or until lightly golden.
While the crust is baking, prepare the filling. In a large bowl, whisk together the eggs, granulated sugar, grapefruit juice, grapefruit zest, salt, and baking powder until well combined.
Once the crust is done, remove it from the oven and pour the grapefruit filling over the hot crust. Return to the oven and bake for an additional 20-25 minutes, or until the filling is set and the edges are lightly golden.
Allow the bars to cool completely in the pan on a wire rack. Once cooled, lift the bars out using the parchment paper and cut into squares. Dust with additional powdered sugar before serving.