This Key Lime Ermine Frosting is light, silky, and packed with bright citrus flavor from fresh key lime juice and zest. It has a smooth, almost whipped texture that feels lighter than traditional buttercream, while still being rich enough to spread or pipe beautifully onto cakes and cupcakes. I absolutely loved it! This is my go-to ermine frosting base, and I love using it for other fruit-forward variations like strawberry ermine frosting, lemon ermine frosting, and orange ermine frosting, too!
Ingredients & Substitutions
- Key lime juice: Gives the frosting its bright, tangy citrus flavor. Fresh key lime juice has the best flavor, but bottled key lime juice works well if you want to save time.
- All-purpose flour: This is what thickens the cooked base and gives ermine frosting its signature silky texture. Make sure to whisk constantly so the mixture stays smooth.
- Unsalted butter: Adds richness and creates the light, fluffy texture once whipped. Room-temperature butter is important so the frosting blends smoothly.
- Granulated sugar: Sweetens the frosting without making it overly heavy. When whipped with the butter, it helps create that classic airy ermine texture.
- Key lime zest: Adds concentrated citrus flavor and makes the frosting taste extra fresh and vibrant. Fresh zest makes a big difference here.
- Vanilla extract: Adds a little warmth and rounds out the tart lime flavor without overpowering it.
- Kosher salt: Just a pinch helps balance the sweetness and brighten the citrus flavor even more.
FAQs
What is ermine frosting?
Also called milk frosting, flour frosting, or $300 frosting, Ermine frosting is a cooked flour frosting made by whisking flour and liquid together into a thick base before whipping it with butter and sugar. The result is light, silky, and less sweet than traditional buttercream. Check out my chocolate ermine frosting and brown sugar ermine frosting, too!
Can I use bottled key lime juice?
Yes. Fresh key lime juice gives the brightest flavor, but bottled key lime juice works well and is a great shortcut when key limes are hard to find.
Why is my frosting too soft?
This usually happens if the cooked lime mixture is still warm when added to the butter. Make sure it cools completely to room temperature before whipping. If needed, chill the finished frosting for 15 to 30 minutes before using.
Can I make this frosting ahead of time?
Yes. You can make it 1 to 2 days ahead and store it in the refrigerator. Before using, let it sit at room temperature for a bit and re-whip until smooth and fluffy again.
Is ermine frosting sweeter than buttercream?
No. Ermine frosting is generally lighter and less sweet than classic American buttercream, which makes it a great option for balancing tart citrus flavors like key lime.
How to Store Key Lime Ermine Frosting
Store the key lime ermine frosting in an airtight container in the refrigerator for up to 4 to 5 days. Because it contains butter and a cooked flour base, it should be kept chilled when not in use. Before using, let the frosting sit at room temperature for 20 to 30 minutes, then re-whip with a mixer until smooth and fluffy again. If it still feels too firm, continue mixing for another minute or two until the texture softens.
You can also freeze the frosting for up to 1 month. Thaw overnight in the refrigerator, then bring to room temperature and re-whip before using.
Key Lime Ermine Frosting
Ingredients
- ¾ cup fresh key lime juice, from about 18 to 24 key limes (or bottled key lime juice)
- 5 tablespoons all-purpose flour
- 1 cup 2 sticks / 227 g unsalted butter, room temperature
- 1 cup (200 g) granulated sugar
- 2 to 3 tablespoons key lime zest, from 3 to 4 key limes
- ½ teaspoon vanilla extract
- 1 pinch kosher salt
Instructions
- n a medium saucepan over medium heat, whisk together the key lime juice and flour until smooth.
- Cook, whisking constantly, until the mixture thickens to a pudding-like consistency and begins to bubble, about 2 to 4 minutes. Continue cooking for 1 additional minute while whisking.
- Remove from heat and transfer the mixture to a shallow bowl or plate. Press plastic wrap directly onto the surface and let cool completely to room temperature.
- In the bowl of a stand mixer with the paddle attachment (or using a handheld mixer), cream the butter and sugar together on medium-high speed until very light and fluffy, about 4 to 5 minutes.
- Add the cooled lime mixture, lime zest, vanilla extract, and salt. Beat on medium speed until combined, then increase to high speed and whip until smooth, light, and fluffy, about 3 to 5 minutes more.
- If the frosting feels too soft for spreading or piping, refrigerate for 15 to 30 minutes before using.
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