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

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

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

Add eggs one at a time, mixing until fully incorporated before adding the next.

Pour in grapefruit juice, zest, and vanilla extract, mixing on low speed until just combined.

Gradually add the dry ingredient mixture to the wet mix, stirring gently until just combined.

Pour the batter into the prepared cake pan, smoothing the top with a spatula.

Bake for 30-35 minutes or until golden brown and a toothpick inserted in the center comes out clean.

Cool the cake in the pan for about 20 minutes, then transfer to a wire rack to cool completely.

Optionally, prepare a simple glaze with powdered sugar and grapefruit juice and spread it over the cooled cake.