Preheat your oven to 350°F (175°C). Grease and line four 6-inch (15cm) round cake pans with parchment paper. Whisk together flour, baking powder, baking soda, and salt. Beat softened butter and sugar on medium-high for 3-4 minutes until fluffy. Add eggs one at a time, then vanilla. Gradually add dry ingredients on low speed until just combined.

Divide the batter equally into two bowls (about 600g each). In one bowl, fold in the grapefruit juice, zest, ¼ cup buttermilk, and pink gel coloring until vibrant pink. In the second bowl, fold in ½ cup buttermilk and the black poppy seeds until pale and creamy.

Pour the pink batter into two pans and the poppy seed batter into the remaining two pans. Bake all four layers for 25-30 minutes until a toothpick comes out clean. Cool for 10 minutes in pans before moving to a wire rack to cool completely.

Beat 1 ½ cups softened butter for 5 minutes until pale. Gradually add powdered sugar on low speed. Add heavy cream and beat on high for 3-4 minutes until bright white and fluffy.

Place one pink cake layer on a plate and spread a thick layer of buttercream. Top with a pale cream poppy seed layer, and repeat the thick frosting. Continue alternating. Frost the top thickly, sprinkle with black poppy seeds, and press two fresh blueberries into the center.