Preheat oven to 350°F. Grease a loaf pan or line it with parchment paper.
Cream together the softened butter and sugar in a large bowl. Separate the egg yolks and add them one at a time, mixing well after each addition.
Zest and juice the organic grapefruit. Add the zest, juice, and vanilla extract to the butter-sugar mixture and mix well.
Combine the flour with the baking powder and poppy seeds. Sift it into the wet ingredients gradually, alternating with a small splash of milk, and mix until a smooth batter forms.
Beat the egg whites with a pinch of salt until stiff peaks form. Gently fold the egg whites into the batter to keep it airy.
Pour the batter into the prepared pan and bake for about 50-60 minutes. Test with a wooden skewer to see if it is baked through. Let it cool completely before removing it from the pan.
Carefully melt the white chocolate over a double boiler or in the microwave. Let it cool slightly.
Beat the cream cheese in a bowl until creamy. Add the melted, cooled chocolate and mix until smooth.
Halve the grapefruit and juice one half. Slice the other half.
Mix the juice into the frosting to taste.
Frost the completely cooled cake evenly with the frosting. Decorate with the grapefruit slices as desired.
