How to Make It
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.
Scrape the batter into the prepared pan and bake for 50 minutes, until the top is golden and a toothpick inserted in the center comes out with moist crumbs attached. Tent the cake with foil halfway through baking to slow the browning. Transfer to a rack to cool for 20 minutes, then unmold and let cool.
In a small microwave-safe bowl, microwave the lemon juice and granulated sugar at high power for 20 seconds, until the sugar is dissolved. Transfer 2 tablespoons of the lemon syrup to a bowl and whisk in the confectioners' sugar and butter. Using a pastry brush, brush the lemon syrup all over the cake. Let stand for 10 minutes to allow the syrup to seep in. Spread the sugar glaze over the cake and let stand until completely dry, 30 minutes.