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

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

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

Stir in the grapefruit juice, zest, and poppy seeds.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

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

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to 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.