Surprise Rainbow Pi Cake

"Foodies and nerds unite to celebrate their love for Pi Day with this fun and colorful lemon pound cake. Slice inside to reveal a surprise multicolor pi symbol."
photo by Arlyn Osborne photo by Arlyn Osborne
photo by Arlyn Osborne photo by Arlyn Osborne
photo by Arlyn Osborne photo by Arlyn Osborne
photo by Arlyn Osborne photo by Arlyn Osborne
photo by Arlyn Osborne photo by Arlyn Osborne
Ready In:
4hrs 30mins
1 loaf cake


  • For the Surprise Pi Symbol

  • nonstick cooking spray
  • 2 (15 1/4 ounce) boxes classic white cake mix (plus required ingredients)
  • deep pink gel food coloring
  • electric green gel food coloring
  • electric yellow gel food coloring
  • purple gel food coloring
  • turquoise gel food coloring
  • For the Lemon Pound Cake

  • 1 12 cups unsalted butter, room temperature
  • 2 14 cups sugar
  • 6 eggs, room temperature
  • 3 lemons
  • 1 12 teaspoons vanilla extract
  • 34 teaspoon lemon extract
  • 3 cups flour, plus more for pan
  • nonstick cooking spray
  • 34 teaspoon baking powder
  • 34 teaspoon salt
  • 14 cup milk, plus 2 tablespoons milk
  • For the Pink Ganache

  • 12 cup heavy cream
  • 1 cup white chocolate chips
  • deep pink gel food coloring
  • For the Decorations

  • 1 (16 ounce) can vanilla frosting
  • turquoise gel food coloring
  • rainbow candy sprinkles


  • Make the Surprise Pi Symbols:

  • Preheat oven to 350 degrees F.  Spray a sheet pan with nonstick spray and line bottom with parchment paper.
  • Prepare the two boxes of cake mix according to package instructions, then divide the batter into five bowls.  Fold in about 6-7 drops of gel food coloring to each bowl, so that you have five separate colors.  Transfer to piping bags.
  • Pipe batter in alternating colors into prepared sheet pan.  Tap the sheet pan on the counter several times to even batter and eliminate any air bubbles.
  • Bake for 20-25 minutes, or until toothpick comes out clean.  Let cool, then freeze until solid.
  • Invert frozen sheet cake onto a cutting board and peel off parchment paper.  Cut pi symbols with cutter and transfer to a parchment-lined baking sheet.  Set aside in freezer.
  • Make the Lemon Pound Cake:

  • Preheat oven to 350 degrees F.  Grease and flour a loaf pan.
  • Whisk together flour, baking powder, and salt in a medium bowl and set aside.
  • In a bowl of a standing mixer fitted with a paddle attachment, beat butter and sugar until light and fluffy.  Add eggs one at a time, beating well after each addition.
  • Add lemon juice, zest, and extracts.  Beat until combined.
  • Add flour mixture and milk, alternating in two batches, beating until combined.  Transfer batter to a large piping bag.
  • Bake the Cake:

  • Pipe and spread a thin layer of batter into the bottom of the prepared loaf pan.
  • Stack three of the frozen pi cakes and position upside down into one side of the loaf pan, leaving a small space on the short side.  Repeat with three more frozen pi cakes, positioning on the other side of the loaf pan.  You should be able to fit six pi symbols inside.
  • Pipe the rest of the batter into the loaf pan, making sure to get batter into all the crevices.  Smooth top with a spatula.
  • Place the loaf pan on a parchment-lined baking sheet and mark which side is the front of the pi symbol.  This ensures you will have a front-facing "pi" when you slice into it.
  • Bake for 1 hour 30 minutes to 1 hour 45 minutes or until a toothpick comes out clean.  If the top is getting too dark during baking, tent with foil.
  • Let cool in pan for 25-30 minutes, then run an offset around perimeter and invert onto a wire rack, making sure to keep track of the front of the cake.  Cool completely.
  • Make Decorations and Assemble:

  • Add the vanilla frosting and several drops of turquoise gel food coloring to a medium bowl.  Fold to combine.
  • Transfer the cooled cake to a turntable.
  • Frost a thin crumb coat all over the cake using a small offset spatula and a bench scraper.  Freeze about 15 minutes, until frosting is hard to the touch.
  • Frost final coating of frosting all over.
  • Put the white chocolate in a medium bowl.
  • Bring the heavy cream to a boil in a small saucepan.  As soon as it comes to a boil, pour over the white chocolate and let sit for 1 minute.
  • Whisk until smooth.
  • Add a few drops of Deep Pink gel food coloring and whisk until combined.
  • Transfer the ganache to a squeeze bottle and let sit for 10-15 minutes to thicken up.  If the ganache gets too thick, microwave the bottle in 10 seconds intervals.
  • Pipe the pink ganache drips all around the sides first.  Then pipe the ganache on top and spread with an offset spatula.  Top with sprinkles.
  • Transfer to a cake stand or platter, keeping note of which side is the front.  Slice and enjoy!

Questions & Replies

  1. Where did you buy the pi cutout? Thanks.
  2. What size loaf pan is this? The cookie cutter used is 4 inches tall from what I can find and the pi symbols fit within the pan, but I can't find a loaf pan deeper than 3 inches. My cutter is 3.2 inches tall, but I can't find a loaf pan it will fit in either.
  3. You refer to the 'front' of the pi sym- bol. Isn't each end of the loaf cake the front. I never got past algebra 2 so I Don't know or remember the pi symbol having a 'front'.


  1. What kind of sprinkles are the best to use if you have a geek in your family and want to make it just for them? I've looked around and couldn't find any sprinkles that were geeky enough. Thanks!



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