Portillo’s Chocolate Cake Recipe

Anyone who’s been to Portillo’s in Chicago knows the drill: you order the Italian beef, you grab a cheese fries, and then somehow a slice of that chocolate cake ends up on your tray. Every single time. Portillo’s chocolate cake is a famously moist, dense Chicago-style chocolate layer cake made with a Devil’s food base, mayonnaise for moisture, and a rich chocolate buttercream frosting, and yes, the mayonnaise is the secret nobody talks about enough. I made this at home for the first time and genuinely couldn’t tell the difference from the real thing.

The Mayo Secret And Why It Works

Before you close this tab hear me out on the mayonnaise. It sounds completely wrong, and yet it’s the smartest thing in this entire recipe. Mayo is essentially just oil and eggs, two things that already appear in most cake batters. Adding it to the mix gives the cake an incredibly moist, tender crumb that holds up for days without drying out.

Portillo’s cake has a reputation for being almost unnaturally moist and this is why. Other chocolate cakes dry out by day two. This one stays soft and fudgy for the better part of a week. Once you understand the science, using mayo in a cake batter feels less bizarre and more like an obvious move you should’ve been making all along.

Ingredients You’ll Need

Straightforward list with one eyebrow-raising ingredient you’ll quickly forgive.

For the chocolate cake

• 1 box Devil’s food cake mix. Portillo’s reportedly uses a box mix as the base, and there’s no shame in that
• 1 cup mayonnaise, full-fat, not light
• 1 cup water
• 3 large eggs
• 1 tsp vanilla extract

For the chocolate buttercream frosting

• 225g unsalted butter, softened to room temperature
• 60g good-quality cocoa powder, sifted
• 400g icing sugar, sifted
• 4–5 tbsp heavy cream
• 1 tsp vanilla extract
• Pinch of salt

FYI, the frosting-to-cake ratio on Portillo’s slices is notoriously generous. Don’t be shy with it. A thin scrape of frosting on a chocolate cake is a missed opportunity. Pile it on, that’s the whole point.

How to Make It Step by Step

Step 1: Mix and bake the cake layers

Preheat your oven to 180°C (350°F). Grease and flour two 9-inch round cake pans. Combine the cake mix, mayonnaise, water, eggs, and vanilla in a large bowl and beat on medium speed for 2 minutes until smooth. The batter will look slightly thicker than standard box mix batter; that’s the mayo doing its thing. Divide evenly between the two pans and bake for 28–32 minutes until a skewer inserted in the centre comes out clean.

Step 2: Cool completely before frosting

Remove from the oven and cool in the pans for 10 minutes, then turn out onto a wire rack. Cool the layers completely for at least 1 hour at room temperature before frosting. Warm cake melts buttercream on contact and you’ll end up with a chocolatey puddle instead of clean, glossy layers. Patience here saves a lot of frustration.

Step 3: Make the buttercream

Beat the softened butter on high speed for 3–4 minutes until pale and fluffy. Add the sifted cocoa powder and beat again until fully combined. Gradually add the icing sugar, alternating with the heavy cream, one tablespoon at a time. Beat in the vanilla and pinch of salt. The finished frosting should be thick, smooth, and spreadable, not stiff, not runny. Add cream by the teaspoon to adjust consistency.

Step 4: Assemble and frost generously

Place one cake layer on your serving plate and spread a thick layer of buttercream across the top; don’t skimp. Place the second layer on top and frost the top and sides completely. IMO, a slightly rustic finish looks more like the real Portillo’s slice than anything overly polished. Press the frosting in broad strokes rather than obsessing over perfection.

Tips for Nailing It Every Time

• Don’t substitute light mayo; the fat content in full-fat mayo is what creates the moisture
• Level your cake layers before stacking if they’ve domed, a serrated knife works perfectly
• Refrigerate the frosted cake for 30 minutes before slicing for cleaner cuts
• Use a hot, dry knife to slice, run it under hot water and dry between each cut
• Bring refrigerated slices to room temperature before serving cold buttercream loses its creaminess

Common Questions

Does Portillo’s actually use mayonnaise in their chocolate cake?

The mayonnaise detail is widely reported and aligns with what makes the cake so distinctively moist compared to standard chocolate cakes. Mayo-based chocolate cakes have been a baking technique since at least the 1930s Hellmann’s actually published a mayo cake recipe in that era. It’s unusual but it absolutely works.

Can I make this cake from scratch instead of using a box mix?

Absolutely. Replace the box mix with a standard Devil’s food cake recipe typically flour, cocoa powder, sugar, baking soda, and buttermilk and add the mayo on top of the wet ingredients. The from-scratch version gives you slightly more control over the flavour depth, especially if you use Dutch-process cocoa.

How long does Portillo’s chocolate cake keep?

Stored in an airtight container, it keeps at room temperature for 3–4 days and stays remarkably moist throughout thanks to the mayo. In the fridge it lasts up to a week, but always bring it back to room temperature before eating for the best texture and flavour.

Final Thought

Portillo’s chocolate cake succeeds because it commits fully to what it is: a gloriously indulgent, unashamedly moist chocolate cake with enough frosting to mean business. The mayo trick isn’t a gimmick; it’s genuinely the reason this cake tastes better the next day than it did fresh out of the oven.

Bake it once for a birthday, a potluck, or honestly just a Tuesday when you want something that makes people stop talking mid-conversation. That’s the Portillo’s effect, and now you can recreate it at home.