Preheat oven to 350ºF. Butter two 4”x 2” round cake pans and line bottoms with parchment rounds.
In a medium bowl, whisk together the flour, cornstarch/arrowroot, baking powder, xanthan gum (if using), baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, or using a bowl and handheld electric mixer, cream together the butter, sugar, vanilla extract, and almond extract on medium-high until light and fluffy (at least 3 minutes). Scrape down bowl and beater as needed.
In a separate bowl, whisk together the egg whites and buttermilk.
Beginning and ending with the dry ingredients, add ¼ of the dry mixture to the butter/sugar mixture, alternating with ⅓ of the liquid ingredients, mixing on low speed and continuing until all parts have been added. Once all ingredients are combined, increased speed to medium and beat for one more minute to fully incorporate. Fold a few times by hand to finish.
Divide batter evenly between the two prepared pans and smooth tops with a spatula – pans will be about ¾ of the way full. Place in preheated oven and bake for 30-35 minutes, until tops are golden-brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Allow cakes to cool in pans completely. Once cool, remove from pans, wrap tightly in plastic wrap, and freeze while preparing frosting.
Chocolate Cake Batter Buttercream
In the bowl of a stand mixer fitted with the wire whip attachment, or using a bowl and handheld electric mixer, beat the butter and cream cheese on medium-high speed until fluffy. Add confectioner’s sugar, slowly mixing in until incorporated and wiping down bowl/beater as needed. Add 3 tablespoons cocoa powder, vanilla extract, almond extract, and salt and mix in. Increase speed to medium-high and beat until smooth and creamy.
Stir with a spatula briefly to get rid of any air bubbles before using. Use this frosting to frost between the cake layers and frost the crumb coating (see assembly and decorating instructions below) before continuing.
Once initial layer of frosting has been added to cake, divide remainder into two equal portions. Add the remaining 4 tablespoons cocoa powder to one half of the frosting, along with the heavy cream. Mix in with a spatula until fully incorporated.
Assembling and Decorating:
Remove cakes from freezer and use a large serrated knife to slice the domed tops off of each so that surface is level.
Place one cake layer on a cardboard cake round set on top of a rotating cake stand. Use an offset spatula or palette knife to spread a layer of frosting over the top. Add second cake layer and cover entire cake with a thin layer of frosting to seal in crumbs. Place in the refrigerator to chill briefly while prepping remaining frosting (see step 3 in buttercream instructions above).
Remove cake from refrigerator and add a very large dollop of lighter frosting color to top of cake. Use offset spatula/palette knife to spread into a flat layer and push excess frosting over edges, working until top of cake is fairly smooth and neat. This frosting should cover the top half of the cake.
Add darker frosting color to bottom half of cake and spread to cover. Use offset spatula/palette knife or bench scraper to smooth out sides of cake while removing excess frosting and gently blending the two colors to achieve an ombré effect. Be careful not to overwork frosting and lose the distinction between the shades.
Use offset spatula/palette knife or bench scraper to smooth out any excess frosting around top of cake. Once done, frosting should be smooth all around.
Decorate cake with remaining frosting, sprinkles, and leftover cake crumbs as desired.*
*I used a French star tip (Ateco 864) for the frosting garnish