Preheat your oven to 350°F (175°C) and prepare an 8-inch square baking pan by lining it with parchment paper.
For the crust, mix together 1 cup of flour, 1/4 cup of sugar, and 1/2 cup of melted butter until crumbly. Press it evenly into the bottom of the baking pan and bake for about 15 minutes until lightly golden.
In a separate mixing bowl, whisk together 2 eggs, 1 cup of sugar, and 1/2 cup of grapefruit juice until well combined, then stir in 1/4 cup of flour.
Pour the grapefruity mixture over the baked crust and return it to the oven for an additional 20-25 minutes, until set and slightly puffed.