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

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

In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing thoroughly after each addition until fully combined.

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

Gradually fold the dry mixture into the wet ingredients until just combined.

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

Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean.

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

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