birthday chocolate cake with lavender ruffle frosting

Ingredients

Special Equipment:

• a candy thermometer, preferably digital

For the Chocolate Cake:

• (makes one 8-inch, three layer cake) 
• 2 cups (9 ounces) all-purpose flour
• 3/4 cup (2.25 ounces) Dutch-processed cocoa powder (I like Hershey's Special Dark)
• 2 teaspoons baking soda
• 2 teaspoons espresso powder
• 1 teaspoon kosher salt
• 1 cup (8 ounces) sour cream, at room temperature
• 2 teaspoons pure vanilla extract
• 1/2 cup (4 fluid ounces) canola oil
• 1 1/2 cups (11.25 ounces) dark brown sugar, tightly packed
• 1/2 cup (3.5 ounces) granulated sugar
• 2 large eggs, at room temperature
• 1 cup boiling water

For the Lavender Swiss Meringue Buttercream Frosting:

• (makes around 6 generous cups — see baker's notes)
• 2/3 cup (6 fluid ounces) egg whites, from about 5 large eggs
• a scant 1 2/3 cup (11 ounces) granulated sugar
• 3/4 teaspoon kosher salt
• 1/4 teaspoon cream of tartar
• 2 1/2 cups (5 sticks // 20 ounces) unsalted butter, at room temperature
• 1 teaspoon pure lavender extract (see baker's notes for sources)
• 1/4 teaspoon almond extract
• purple food coloring 

Recipe

For the Chocolate Cake:

1. Center a rack in the oven and preheat to 350 (F). Prepare three 8-inch cake pans by spraying generously with cooking spray and lining the bottoms with a parchment paper circle round; spray the parchment paper as well and set aside.
2. In a medium bowl, whisk together 2 cups all-purpose flour, 3/4 cup Dutch-processed cocoa powder, 2 teaspoons baking soda, 2 teaspoons espresso powder, and 1 teaspoon kosher salt. Set aside.
3. In a large bowl, whisk together 1 cup sour cream, 2 teaspoons pure vanilla extract, 1/2 cup canola oil, 1 1/2 cups tightly packed dark brown sugar, 1/2 cup granulated sugar and 2 large eggs until very well combined. 
4. Sprinkle the dry ingredients (from the 2nd step) over the wet ingredients (from the 3rd step) and whisk until the dry ingredients are just incorporated into the wet ingredients — don't worry about any lumps or streaks of dry ingredients at this point. Whisk in 1 cup boiling water, 1/4 cup at a time, until all of the water is incorporated and you have a nice, smooth batter — the batter will be a little thin, but that's okay. Divide the batter evenly between the prepared pans.
5. Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center of each cake comes out with just a few crumbs. Transfer to a wire rack and cool completely before frosting.

For the Lavender Swiss Meringue Buttercream Frosting:

1. Fill a medium sauce pot with at least 1 1/2 inches of water and place over medium-high heat until steaming hot, then adjust the temperature to maintain a gentle simmer. 
2. Combine 6 ounces egg whites, 1 2/3 cups granulated sugar, 3/4 teaspoon kosher salt, and 1/4 teaspoon cream of tartar in a medium, heatproof glass bowl and whisk to combine. Place the glass bowl over the medium sauce pot to create a homemade bain-marie, ensuring that the bottom of the glass bowl does not touch the steaming water. Fix a candy thermometer to the side of the bowl and use a rubber spatula to occasionally stir and scrape down the egg white mixture from the sides of the bowl as it warms to 185 (F). This should only take around 10 to 12 minutes, so if the mixture is moving slowly, don't be afraid to turn up the heat!
3. Once the egg white mixture reaches 185 (F), transfer immediately to the bowl of a freestanding electric mixer fitted with a whisk attachment. Turn on the mixer to its highest setting and whip the egg whites for 10 minutes to create a glossy, stiff meringue that is cool to the touch (around 90 (F)). 
4. Once you've made the meringue, turn off the mixer and use a rubber spatula to scrape any excess meringue off the whisk attachment and back into the bowl. Swap out the whisk attachment for the paddle attachment and turn the mixer back on to a medium speed. Add 2 1/2 cups unsalted butter slowly, adding 1 or 2 tablespoons to the mixture at a time. Initially, the volume of the meringue will decrease dramatically and it will seem like the mixture is too liquidy — but as you add more butter, the mixture will thicken and cool. In the end, the buttercream should be thick, creamy, and soft but not runny. 
5. Reduce the mixer speed back to its slowest setting and add 1 teaspoon pure lavender extract, 1/4 teaspoon almond extract, and purple food coloring until the buttercream is a uniform color throughout. Use buttercream right away; even though the buttercream will look like it's too soft, it's actually a dream to pipe with. Unless it's a super hot day (above 85-90 (F)), there's no need to refrigerate it for it to stiffen — doing so will just make it harder to work with.