Preheat oven to 325°. Mist a 10-inch Bundt or tube pan with cooking spray.
In a large bowl, combine flour, baking powder, and salt. In a separate bowl, beat butter with an electric mixer until color lightens, about 2 minutes. Add sugar gradually; then beat until fluffy, about 4 minutes. Add grapefruit zest and extract (if you like); beat thoroughly.
Add eggs, one at a time, beating well after each addition. Add one-third of the flour mixture, then beat until just incorporated. Add ¼ cup grapefruit juice and another third of the flour mixture; then beat until just incorporated. Repeat with the other ¼ cup grapefruit juice and the remaining flour mixture.
Pour batter into the prepared pan. Tap pan on counter to get rid of any air pockets. Bake 50 minutes, or until a toothpick inserted near the center comes out clean.
Remove cake from oven and place on a cooling rack 10 minutes; then turn cake out of the pan onto rack. Loosen sides with a thin knife. Let cool to room temperature, about 45 minutes longer. Top with Grapefruit Glaze as directed below.
Combine sugar and juice in a bowl; then mix with a fork until smooth. Set a piece of wax paper under the cake (which is still on the cooling rack); then slowly drizzle glaze over cake. Let cake cool completely before transferring to a platter. You may refrigerate it overnight, or up to several days, covered loosely with foil.