Preheat your oven to 350°F (175°C) and prepare a 9-inch cake pan by greasing and flouring it.

In a large mixing bowl, whisk together flour, sugar, baking powder, and salt until combined.

In another bowl, beat eggs with milk and vegetable oil. Fold in grapefruit zest and juice.

Pour the wet mixture into the dry ingredients and stir gently until just combined, then fold in poppy seeds.

Transfer batter into the prepared cake pan and bake for 30-35 minutes or until a toothpick comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a bowl, whisk powdered sugar with grapefruit juice until smooth, adjusting the consistency as needed.

Once the cake is cooled, drizzle the glaze over the top and slice to serve.