Preheat the oven to 350°F (175°C) and prepare an 8-inch round cake pan by greasing and dusting with flour.

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

In a large bowl, cream together the butter and sugar until light and fluffy, about 3-5 minutes.

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

Mix in grapefruit juice, zest, and vanilla extract until combined.

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

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

Bake for 30-35 minutes. Check for doneness using a toothpick.

Allow the cake to cool in the pan for about 20 minutes, then turn out onto a wire rack.

For optional glaze, mix powdered sugar with grapefruit juice and drizzle over the cooled cake.