Step 1

Adjust oven rack to lower-middle position and heat oven to 350 degrees. Grease two 9-inch round cake pans, line bottoms with parchment paper, and grease parchment.

Step 2

Microwave oil, chocolate, and cocoa together in bowl at 50 percent power, stirring occasionally, until melted, about 2 minutes. Whisk mixture until smooth, then set aside to cool slightly. In separate bowl, whisk flour blend, baking powder, baking soda, xanthan gum, and salt together.

Step 3

In large bowl, whisk eggs and vanilla together. Whisk in sugar until well combined. Whisk in cooled chocolate mixture and milk until combined. Whisk in flour blend mixture until batter is thoroughly combined and smooth.

Step 4

Divide batter evenly between prepared pans and smooth tops. Bake until toothpick inserted into center of cake comes out clean, 30 to 32 minutes, switching and rotating pans halfway through baking.

Step 5

Let cakes cool in pans on wire rack for 10 minutes. Run knife around edge of cakes to loosen. Remove cakes from pans, discard parchment, and let cool completely on rack, about 1 1/2 hours. (Cake layers can be wrapped tightly in plastic wrap and stored at room temperature for up to 1 day.)

Step 6

Place 1 cake layer on platter and spread 1 1/2 cups frosting evenly over top using small icing spatula or butter knife. Top with second cake layer, press lightly to adhere, then spread remaining 2 1/2 cups frosting evenly over top and sides. Serve.