1. Center a rack in oven and heat to 350 degrees. Generously grease and flour a 5x9" loaf pan.
2. Whisk flour, baking powder, baking soda, and salt in a medium bowl. In a large bowl, beat butter with an electric mixer on medium-high until creamy, then add 3/4 cup granulated sugar and 1 1/2 tablespoons grapefruit zest and beat until light and fluffy, about 2 minutes. Scrape sides of bowl, then reduce speed to medium and beat in eggs one at a time.
3. Reduce speed to low, then beat in one-third of flour mixture until mostly combined, followed by 1/4 cup grapefruit juice. Mix in another third of flour mixture, then add buttermilk. Add remaining flour mixture and mix until just blended. Transfer to prepared pan and smooth top, then bake until a toothpick inserted in center comes out clean, 40 to 50 minutes. Set pan on a wire rack while you make the syrup.
4. Place 1/3 cup juice in a small saucepan along with remaining 1/4 cup granulated sugar and bring to a boil over medium, stirring to dissolve sugar; remove from heat. Use a bamboo or metal skewer to poke holes all over the cake, spaced about 1/2" apart, then slowly pour syrup over top. Cool in pan 30 minutes, then loosen edges with a knife and gently turn cake out of pan; let cool completely on rack.
5. Transfer cake to a serving plate. Whisk confectioners' sugar, remaining 1/2 tablespoon grapefruit zest, and 1 tablespoon juice in a small bowl until smooth. (Glaze should be thick but pourable; add more juice as needed to reach desired consistency.) Drizzle glaze over cake, spooning any excess that pools on plate back over top, then let sit 15 minutes before serving.
Source: Hannaford fresh Magazine, January-February 2021