Preheat the oven to 350°F. Butter a 9-x-5-inch loaf pan.
Add 1⁄2 cup of the grapefruit juice to a small saucepan set over medium heat. Bring to a simmer and reduce the juice by half. Cool slightly.
In a medium bowl, combine 1 tablespoon grapefruit zest, yogurt, eggs, olive oil, and reduced grapefruit juice. Whisk to mix well.
In a large bowl, add the granulated sugar, flour, baking powder, baking soda, and salt. Whisk to combine.
Add the wet ingredients to the dry ingredients. Mix until everything is well blended. Stir in the chocolate.
Pour the batter into the prepared pan and place in the hot oven. Bake for 50 to 55 minutes, until the cake is brown, set and springs back gently when pressed.
While the cake is baking, prepare the glaze. In a bowl, combine the remaining 1⁄2 tablespoon grapefruit zest with the remaining 1⁄4 cup of grapefruit juice. Slowly add the confectioners' sugar while stirring. Continue until well mixed.
Let the cake cool in the pan for 5 minutes before removing the cake and cooling on a wire rack.
When cooled to room temperature, place the cake on a serving platter and drizzle with half the glaze. Reserve the rest of the glaze for serving along with the sliced cake. Serve with crème fraîche, if desired.
The cake can be made 1 day in advance.