From: Art of Gluten-Free Baking (http://artofglutenfreebaking.com)
-adapted from Vintage Cakes by Julie Richardson
Yield: 1- 3 layer cake
For the Cake
For the Berry Buttercream
For the Sugar Syrup
For the Ganache
Make the cake
In a medium bowl, pour the boiling water over the unsweetened chocolate and cocoa and let sit for 1 minute. Whisk until smooth. Add the sour cream and vanilla. Whisk well to combine.
Sift together flour, baking soda, and salt in a small bowl.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and both sugars on medium-high until light and fluffy—about 3 minutes. Scrape down the bowl with a rubber spatula. On low speed, drizzle oil into the mixture until it’s blended. On low speed, add the egg yolks and eggs, one at a time, until well blended. On low speed, add the flour mixture and chocolate mixture alternately, beginning and ending with the flour mixture. Blend until smooth.
Divide the batter equally between the 3 pans (there will be approximately 510 g of batter per pan). Smooth tops and tap each pan on the counter to release and large air bubbles.
Bake at 350 degrees F for about 25 minutes—until the centers of the cakes spring back when lightly touched. Place pans on wire racks to cool for about 10 minutes, then unmold onto wire racks to cool completely.
When the cakes are completely cool, wrap each tightly in a piece of plastic wrap and place in the freezer for at least an hour or overnight.
Make the ganache
Make the Berry Buttercream
In the bowl of a stand mixer, place egg whites, sugar, and cream of tartar. Use a hand whisk to combine. Place bowl over (not in) a pan of simmering (not boiling) water and whisk constantly. As the mixture warms up, it will become easier to whisk. Continue whisking until the side of the bowl is hot to the touch.
Carefully move the bowl to the stand mixture and using the whisk attachment, whip the whites on medium high until they have tripled in volume and are glossy and they hold stiff peaks, 3-4 minutes. Turn the speed to low and add butter pieces, one at a time. Add each piece after the piece before it has been incorporated into the mixture. The mixture may go through a phase where it looks curdled—that’s OK, it is normal and will eventually smooth out. Once all of the butter is incorporated and the frosting looks smooth, add the mashed and strained berries, vanilla, and salt until combined.
You can now cover with plastic wrap and make your ganache. If you’re not using it fairly soon, the buttercream will keep at room temperature for about 2 days or in the fridge for about 7 days. If you place it in the fridge, you will need to bring it to room temperature. Either way, the buttercream must be re-whipped, either by hand or in the bowl of a stand mixer fitted with the whisk attachment.
Make the sugar syrup
Frost and assemble the cake
Tip: to save on clean up time, place cut pieces of waxed or parchment paper around the edges of the bottom cake layer—shoving them under the cake. This way, the paper catches frosting/ganache drips. Then when you’re done frosting the cake, remove the paper pieces—and you have a clean plate!
Using a pastry brush, brush the top with the sugar syrup.
Next, spread a thin layer of the ganache on top.
Next, spread with about ¾ cup of the buttercream. I like to do this with a metal offset spatula. The buttercream will be about ¼ in/.66 cm thick.
Unwrap the next cake layer and stack bottom up on top of the first layer. Repeat the sugar syrup, ganache, and buttercream process.
Unwrap the final cake layer and stack on the top of the second layer. Repeat the sugar syrup and ganache process. Now, check your cake and carefully scrape off any ganache or buttercream that has oozed out of the layers. Then, apply a thin coating of buttercream over the entire cake. You should be able to see parts of the cake through the buttercream. This is your “crumb coat.” What this does is contain any loose crumbs that are present on the cake so they don’t get mixed up into your final frosting and make it look messy.
Place the cake into the refrigerator for about 10 minutes to firm up the crumb coat before you do your final frosting. After 10 minutes, remove from fridge and carefully frost the cake with the remaining buttercream.
Store the cake in an airtight container (I like to use a cake carrier) for up to 3 days.