Preheat your oven to 350°F (175°C) and prepare an 8-inch round cake pan by greasing it with butter or non-stick spray and dusting with flour.

In a medium mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and poppy seeds until well combined.

In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer until light and fluffy, about 3-5 minutes.

Add the large eggs to the butter-sugar mixture, beating well after each addition until fully incorporated.

Mix in the grapefruit juice, grapefruit zest, and vanilla extract until the mixture is smooth and well combined.

Gradually add the dry ingredients into the wet mixture, folding gently until just combined and no flour pockets remain.

Transfer the batter to the prepared cake pan, smoothing the top for even baking.

Bake in the preheated oven for 30-35 minutes, checking for doneness with a toothpick.

Allow the cake to cool in the pan for 20 minutes before transferring it to a wire rack to cool completely.

If desired, frost the cake with a light glaze made from powdered sugar and grapefruit juice or leave it plain.