For the Cake:
6 oz. (3/4 cup) unsalted butter, softened; more for the pans
8 oz. (1-3/4 cups) all-purpose flour; more for the pans
2 cups packed dark brown sugar
2 tsp. pure vanilla extract
3 large eggs, at room temperature
2-1/4 oz. (3/4 cup) unsweetened Dutch-processed cocoa powder
1-1/4 tsp. baking soda
1 tsp. baking powder
1 tsp. kosher salt
1-1/2 cups buttermilk, preferably low fat, at room temperature
1/4 cup mayonnaise
Position a rack in the center of the oven and heat the oven to 350°F. Butter two 8x2-inch round cake pans and line each with a parchment round. Butter the parchment, dust with flour, and tap out any excess.
In a stand mixer fitted with the paddle attachment, beat the butter, brown sugar, and vanilla on medium-high speed until lighter in color and slightly increased in volume, 3 to 5 minutes. Lower the speed to medium and add the eggs, one at a time, mixing until each is fully incorporated before adding the next.
Sift the flour, cocoa powder, baking soda, and baking powder onto a piece of parchment. Add the salt to the dry ingredients after sifting.
Using the parchment as a chute, add one-quarter of the dry ingredients to the batter and mix on low speed until incorporated. Add about 1/2 cup of the buttermilk and mix on low speed until incorporated. Continue to alternate dry ingredients and buttermilk, mixing until incorporated after each addition and stopping to scrape the bowl and beater as necessary. Using a whisk, fold the mayonnaise into the batter.
Divide the batter evenly between the prepared pans and bake until a toothpick inserted in the center of the cakes comes out clean and the sides of the cake have begun to pull away from the pan slightly, 40 to 45 minutes. Remove the pans from the oven and cool on a rack for 15 minutes. Invert the cakes onto the rack and remove the pans and parchment. Cool the cakes completely.
For a printable version of this recipe, go here.
For the Buttercream
5 large egg whites
1-1/2 cups sugar
4 sticks unsalted butter, diced and softened
Pinch of salt
1 tablespoon vanilla
Combine egg whites and sugar in a heat-proof bowl placed over a saucepan of simmering
water (i.e. bain marie). (Take care to ensure the bottom of the bowl does not touch
the simmering water, but rests a little above it.) Bring mixture to 160 degrees F while
whisking constantly. (I use the metal bowl of my stand mixer placed over a saucepan;
that way, I only dirty one bowl).
Transfer mixture to stand mixer bowl (or, if you’re already using your mixer bowl,
transfer it to the stand mixer), fitted with a whisk attachment and beat on medium speed
until mixture cools and doubles in volume. Depending on your mixer and other variables,
the time this takes varies. I find it usually takes around 10 minutes, give or take a few.
Add butter in one piece at a time, mixing to incorporate after each addition. The mixture
may appear clumpy and almost curdled looking, but as you continue to mix it, it will
correct itself.. Add salt and vanilla and mix to combine thoroughly.
You want a smooth, satin-like finish. If it’s runny or soupy, add a few more tablespoons
of chilled butter (not straight out-of-the-fridge hard and cold, but colder than room
temperature). If it appears curdled, just keep beating it until it comes back together.
You can refirgerate this when you’re done, but you must bring it to room temperature
again before trying to frost a cake or cupcakes with it. You can also freeze it (I put mine
in a Ziploc freezer bag).
NOTES:
You will find slight variations in different Swiss Meringue Buttercream recipes,
but it’s a pretty classic baker’s standby. I have tried several different variations
and this recipe is my favorite.
In this recipe, I prefer freshly separated egg whites over pasturized egg white
product.
Make sure you use unsalted butter. And be sure your butter is at room temperature
before incorporating it into the meringue; if it’s too cold or too firm, it won’t
blend properly. Likewise, if the butter is too warm, it won’t come together
properly either.
This recipe takes all kinds of flavor additions very well (extracts, fruit curds,
ganache, etc.). Experiment!
Keep your buttercream out of the heat and sun. It will melt and turn greasy.
For a printable version of this recipe, go here.