Preheat your oven to 350°F (175°C) and grease an 8×8-inch baking dish.

In a mixing bowl, combine melted butter and sugar until smooth. Mix in the flour until crumbly, then press into the bottom of the prepared dish.

In another bowl, whisk together eggs, grapefruit juice, zest, and remaining sugar until well combined. Pour this mixture over the crust.

Bake for 20-25 minutes, or until set but still slightly jiggly in the center.

Let cool completely before slicing into squares.