This grapefruit poppy seed cake is bright, bakery-style, and perfect when you want a citrus dessert that feels elegant—not heavy. The layers are fluffy and tender, the grapefruit curd adds a tangy punch, and the cream cheese frosting brings everything together with a soft, creamy finish.

Ingredients (makes 1 tall 8-inch layer cake)

1) Poppy Seed Cake Layers

• 1 cup (226 g) unsalted butter, room temperature
• 1 3/4 cups (350 g) granulated sugar
• 6 egg whites, room temperature (save 3 yolks for the curd)
• 1 cup (240 g) sour cream, room temperature
• 1 tsp (5 g) vanilla extract
• 3 cups (345 g) cake flour
• 1 tbsp (10 g) baking powder
• 1 tsp (5 g) salt
• 1 cup (230 g) grapefruit juice (fresh squeezed is best)
• 3 tbsp (27 g) poppy seeds (soak a few minutes, then strain and dry)

2) Grapefruit Curd

• 3 egg yolks
• 1/2 cup (100 g) sugar
• 2 tbsp (16 g) cornstarch, sifted
• 1 cup (230 g) grapefruit juice
• 2 tbsp (28 g) unsalted butter, cold
• Optional: 1 drop “soft pink” gel color (for a pinker curd)

3) Cream Cheese Frosting

• 1 1/2 cups (339 g) unsalted butter, slightly chilled
• 4 oz (113 g) cream cheese, slightly chilled
• 6 cups (750 g) powdered sugar, measured then sifted
• 3 tbsp (42 g) heavy whipping cream
• 1 tsp (4 g) vanilla extract
• Pinch of salt

Step-by-step

A) Prep

1. Soak the poppy seeds in water for about 5 minutes, then strain and pat dry (helps prevent a dry cake).
2. Preheat oven to 325°F / 163°C.
3. Grease three 8-inch pans (or four 6-inch pans), line bottoms with parchment, then grease again.

B) Bake the cake layers

1. In a bowl, whisk together cake flour + baking powder + salt + poppy seeds.
2. Cream butter + sugar on medium-high for about 2 minutes until lighter.
3. Add the egg whites gradually, mixing and scraping the bowl as you go.
4. Mix in sour cream until combined (about 1 minute).
5. On low speed, alternate adding:
   • 1/3 dry mix → 1/2 cup grapefruit juice
   • 1/3 dry mix → 1/2 cup grapefruit juice
   • final 1/3 dry mix
     Then add vanilla and mix just until smooth (don’t overmix).
6. Divide batter evenly into pans (about 18 oz per 8-inch pan).
7. Bake 28–30 minutes, until a toothpick comes out with a few moist crumbs (not wet batter).
8. Cool 15 minutes in pans, then turn out onto racks to cool completely.

Tip: If you can, wrap and chill/freeze layers before assembling—cold layers stack cleaner.

C) Make the grapefruit curd

1. In a saucepan, whisk sugar + cornstarch together (a small pinch of salt is fine too).
2. Whisk in grapefruit juice and cook over medium heat, stirring until it begins to thicken.
3. Temper the yolks: slowly whisk a small amount of the hot grapefruit mixture into the yolks (whisking constantly), then pour the warmed yolks back into the saucepan.
4. Bring to a gentle boil and cook a few minutes until thick and glossy.
5. Remove from heat, whisk in cold butter until smooth.
6. Strain through a fine mesh sieve for a silky curd.
7. Cover (press plastic wrap directly on the surface) and chill until cold and thick.

D) Make the cream cheese frosting

1. Beat butter + cream cheese until light and fluffy.
2. Add powdered sugar gradually (about 1 cup at a time), mixing and scraping the bowl.
3. Beat in vanilla + pinch of salt + heavy cream, then whip a few minutes until airy.

Note: This frosting can be a bit “sticky” (harder to get perfectly sharp, smooth sides). A softly textured/rustic finish looks great.

E) Assemble the cake

1. Place 1 cake layer on a board/plate.
2. Pipe a ring of frosting around the edge (a “dam”) to hold the curd in.
3. Spread a thin layer of frosting inside the ring, then spoon about 1/4 to 1/3 cup curd on top (don’t overfill or it may ooze).
4. Add the second layer and repeat.
5. Add the final layer, crumb coat, chill 15–20 minutes, then frost fully.
6. Optional: garnish with a little grapefruit zest, thin grapefruit slices, or a light sprinkle of poppy seeds.

Storage

• Keep refrigerated (because of cream cheese + curd).
• Best texture if you let slices sit at room temp 15–20 minutes before serving.