Preheat oven to 350°F (180°C). Line 8 x 8-in (20 x 20-cm) square baking pan with parchment paper, allowing extra for overhang.
In large bowl, beat together 3/4 cup (175 mL) butter and 1/4 cup (60 mL) sugar. Next, beat in 3/4 cup (175 mL) flour and 1/4 tsp (1 mL) salt until combined. Gradually mix in remaining 3/4 cup (175 mL) flour, using hands if necessary, to form a smooth dough. Press dough evenly into prepared pan. Bake 30 to 35 min., or until golden brown. Cool on rack.
Meanwhile, in small saucepan, whisk together eggs, egg yolks and remaining 1 1/4 cups (300 mL) sugar. Whisk in grapefruit zest and juice and remaining 1/4 tsp (1 mL) salt. Set saucepan on medium heat; stir constantly 5 to 8 min., or until curd is thick enough to coat back of spoon. Immediately remove from heat. Strain curd through fine mesh strainer into clean bowl. Cut remaining 1/3 cup (75 mL) butter into chunks. Whisk into curd.
Pour curd over cooked shortbread base; spreading evenly. Bake 15 min., or until curd is just set but still slightly wobbly in middle. Cool on rack to room temperature; cover and chill 2 hr., or overnight. Dust with icing sugar. Using overhanging parchment, remove from pan and slice into 20 bars.
Replace grapefruit zest and juice with lemon zest and juice for lemon curd variation of the bars.