Set rack in the middle of the oven and heat the oven to 350 degrees. Prepare an 8x8 inch baking pan using Smitten Kitchen's "sling" method. Cut 2 pieces of parchment the same width as the bottom of your pan, but longer on each end so that when you lay the parchment in the pan, extra parchment paper hangs over both sides. Lay the other piece of parchment going in the opposite direction of the first.
Beat together the butter and sugar on high speed until light and fluffy, about 3 minutes, scraping the sides occasionally. Add the flour and salt and mix on low speed until combined. Transfer the dough to the bottom of the pan and use your fingers to press it evenly into the space.
Bake the dough until lightly browned about 15-18 minutes. Remove from oven and set aside.
Meanwhile, make the filling. In a medium bowl add the eggs and sugar. Whisk vigorously for about 1 minute until slightly thick. Add the flour, grapefruit juice and zest. Whisk well and pour the filling over the warm baked crust.
Bake until lightly browned on top and no longer jiggly in the center, about 18-20 minutes. Cool completely in the pan. Lift the parchment paper and transfer the cookie to a cutting board. Use a long, thin knife to cut the bar into 16 squares.
Store in a airtight container, separating layers with a piece of parchment or wax paper -- so they don't stick together.