Preheat your oven to 350°F (175°C). Prepare two 8-inch cake pans by spraying them with nonstick spray and lining the bottoms with parchment paper.

In a medium bowl, whisk together the granulated sugar, oat flour, mochiko, salt, baking powder, baking soda, and xanthan gum until well combined.

In a large bowl, combine the cocoa powder, vegetable oil, and boiling water, and whisk until the cocoa is fully dissolved.

Add the eggs, buttermilk, and vanilla; whisk until all the wet ingredients are mixed.

Gradually mix the dry ingredients into the wet ingredients until you achieve a smooth batter.

Divide the batter evenly between the prepared pans.

Bake the cakes for 22 to 28 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow the cakes to cool in the pans for about 10 minutes, then transfer them to a wire rack to cool completely.

In a bowl, whisk together powdered sugar, cocoa powder, vanilla, salt, and boiling water until smooth, and set aside to cool.

Melt the chocolate using a double boiler method over simmering water until it becomes smooth, then allow to cool slightly.

In a separate mixing bowl, beat the room temperature butter into the cooled sugar mixture until smooth.

Add the melted chocolate and continue beating until everything is well combined. Chill in the fridge until thickened if necessary.

Once the cakes are completely cool, carefully turn them out of the pans and remove the parchment paper.

Stack the cakes on top of one another with a generous layer of frosting between each layer and cover the top and sides of the cake with the remaining frosting.

Chill the assembled cake in the fridge until firm enough to slice.