Line the bottom of two, 9 inch cake pans with parchment paper. Lightly grease the parchment and the edges of the pans and dust with gluten free flour.
Preheat the oven to 350° F.
Sift together the gluten free 1 to 1 flour, tapioca, baking powder, baking soda, salt, and cocoa powder into a medium bowl. Repeat. Set aside.
Beat the butter and sugar in the bowl of a stand mixer, on medium speed, until starting to cream. Add the oil and continue mixing until fluffy and creamy.
Add the eggs, one a time, then the yolk, beating after each. Add in the vanilla and stir in. Mix in the flour mixture and dry ingredients, alternately in 3-4 portions. Mix until just combined and smooth.
Pour in the boiling water and mix on low speed until combined.
Spoon into the prepared pans and smooth the top. Bake in the center of the oven for 20-25 minutes or until the cake springs back when pressed with a finger, and a toothpick comes out clean when inserted into the center. The cake will be puffy and starting to brown on the edges.
Let the cake cool the pan for 15 -20 minutes, before turning out onto a wire rack to finish cooling.
Frost with chocolate or vanilla frosting and pipe decoration or top with edible, washed organically grown flowers or dairy free gluten free sprinkles. Makes 2 nine" cakes. 16 servings.