Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, cream together the sugar and softened butter until light and fluffy.

Add the eggs one at a time, beating well after each addition.

In another bowl, whisk together the flour, poppy seeds, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the creamed mixture, alternating with buttermilk, grapefruit juice, and zest. Mix until just combined.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 30-40 minutes or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.

In a small bowl, whisk together the powdered sugar and grapefruit juice until smooth.

Drizzle the glaze over the cooled cake before serving.