Make the Cake
Preheat the oven to 350°. Butter and flour a 9 1/2-by-5-inch glass loaf pan. In a medium bowl, whisk the flour, baking powder, baking soda and salt. In a small bowl, whisk the grapefruit juice with the yogurt. In a medium bowl, using an electric mixer, beat the butter with the sugar at medium-high speed until fluffy. Beat in the eggs and lemon zest. Beat in the dry and wet ingredients in 3 alternating additions; scrape down the bowl as necessary.