Preheat the oven to 350˚F.
Line a 9 by 5-inch loaf pan with parchment paper.
Whisk the flour, baking powder, salt, and cinnamon in a bowl.
In a second bowl, cream the butter with the sugar until light and fluffy. Add the eggs and yolk, stirring well. Stir in the yogurt until fully incorporated.
Fold in the flour mixture until fully incorporated. Stir in the grapefruit zest and juice, vanilla, and poppy seeds just until combined.
Spread the batter evenly into the prepared pan and bake until a toothpick comes out clean when inserted into the center, 90 to 105 minutes.
Remove from the oven and let cool completely in the pan, about 1 hour, before slicing and serving.