I developed this frosting to bring the sophisticated flavor of beurre noisette to Italian buttercream. It's not a frosting to use on a child's birthday cake, but it should be hit at your next party for grown-ups . Bananas complement beurre noisette, so I've added an optional banana cake filling to the recipe.

The day before:

4.5 sticks (36 tablespoons) unsalted butter

Cook 2 sticks (16 tablespoons) of butter to the beurre noisette stage, remove from heat and place on ice or in a cold water bath. Stir for about 20 seconds to halt cooking. Add 2.5 sticks (20 tablespoons) of butter, in thirds, melting the butter with each addition. It will take a while to get all of the butter melted. Pour into an air-tight container, scraping the bottom of the pan to include the pieces of caramelized milk solids, and refrigerate overnight.

Beurre noisette icing:

5 egg whites from large eggs at room temperature
.5 teaspoon cream of tartar
.25 cup (48 grams) white sugar
1 cup (192 grams) demerara sugar (I use Domino brand.)
1 teaspoon (3 grams) kosher salt (or .5 teaspoon [3 grams] table salt)
.25 cup water
beurre noisette mixture from the day before, at room temperature
2 teaspoons vanilla extract

Remove from the refrigerator the beurre noisette mixture you cooked the day before and allow it to warm to room temperature. It will take some time to reach room temperature and it must be soft enough to scoop when it comes time to make the icing. This is especially important if you are working in a cool environment where it will take extra time for the butter to reach room temperature.

Using a stand mixer with a whip attachment, whip 5 egg whites and .5 teaspoon cream of tartar on medium-high speed until soft peaks form. With the mixer on, slowly pour in the white sugar. Continue whipping until the mixture forms glossy, stiff peaks but is not dried out. Turn the mixer off.

In a sturdy saucepan fitted with a candy thermometer, combine 1 cup demerara sugar, .25 cup water and the 3 grams of salt. Place over a medium-high flame and cook until the thermometer reads 245 degrees Fahrenheit.

When the sugar syrup is cooked, turn the stand mixer back to medium-high speed for a few seconds and turn it off again. Add a small amount of the sugar syrup and turn the mixer back on. Continue doing this until all of the sugar syrup is incorporated into the egg whites. Then, turn the mixture to its highest speed and beat until the outside of the bowl is barely warm to the touch.

Take a kitchen tablespoon and, with the mixer still on, scoop out approximately one tablespoon of the beurre noisette mixture and drop it into the mixer. (It doesn't need to be an exact tablespoon.) Wait until the beurre noisette is fully incorporated into the egg whites, and drop another tablespoon. Keep doing this until all of the beurre noisette is incorporated.

With the mixer still running, incorporate 2 teaspoons of vanilla extract and make sure it is evenly distributed in the icing. (Turn off the mixer and scrape the sides and bottom, if necessary.)
If making the filling, put the icing in a bowl and set aside.

Cake filling (optional):

1 over-ripe banana (I usually buy a week ahead and allow it to ripen.)
1.5 cup (180 grams) powdered sugar
.5 teaspoon fresh lemon juice
1 cup beurre noisette icing

Using the stand mixer with the paddle attachment, combine the banana, 1.5 cups of powdered sugar and .5 teaspoon of lemon juice and beat on medium speed until fully incorporated. Switch to the whisk attachment and add the cup of beurre noisette icing. Whip on high speed until fully incorporated and airy. Fill the cake leaving about a half inch margin around the sides, to prevent the filling from oozing out.