Preheat your oven to 350°F (175°C) and grease an 8-inch round cake pan with non-stick spray or butter, then dust it with flour.

In a large bowl, whisk together flour, baking powder, baking soda, salt, and poppy seeds.

In a separate bowl, beat the butter and sugar together until pale and fluffy, about 3-4 minutes.

Add the eggs to the butter-sugar mixture one at a time, mixing well after each addition.

Pour in the grapefruit juice, zest, and vanilla extract; mix gently until combined.

Fold the dry ingredients into the wet mixture until just combined.

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

Bake for 30-35 minutes until a toothpick inserted comes out clean.

Cool the cake in the pan for 20 minutes before transferring to a wire rack.

Whisk together powdered sugar and grapefruit juice for the glaze, then drizzle over the cooled cake.