Preheat oven to 350°F (175°C). Grease, flour, and line four 8-inch cake pans with parchment paper.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large bowl, rub the grapefruit zest into the granulated sugar until fragrant. Add the butter and oil, and beat until light and fluffy.

Beat in the eggs one at a time, followed by the vanilla extract.

Alternate adding the dry ingredients and the buttermilk, mixing on low speed just until combined.

Divide the batter evenly in half. Fold poppy seeds into one half. Fold grapefruit juice and pink food coloring into the other half.

Pour the batters into the prepared pans (two pink layers, two poppy seed layers). Bake for 22-26 minutes until a toothpick comes out clean.

Cool cakes in pans for 10 minutes, then invert onto wire racks to cool completely.

For the frosting: Beat cream cheese and butter until smooth. Gradually add powdered sugar and vanilla, beating until fluffy.

Assemble the cake by alternating the pink grapefruit and yellow poppy seed layers, spreading frosting between each layer. Frost the outside and garnish with berries and extra poppy seeds.