Preheat the oven to 350°F. Spray with cooking spray or lightly grease two 9″ round cake pans, and line with parchment paper.
Sift or whisk together the brown rice flour, potato starch, tapioca flour, baking powder, xanthan gum, and salt. Set aside.
In a separate bowl, use an electric mixer or stand mixer to cream the unsalted butter and sugar.
Add 1 egg, and beat for a minute until fluffy. Then add the remaining eggs, one at a time, beating after each addition.
Scrape down the sides of the bowl. Add the pure vanilla extract and beat until smooth.
Reduce the mixer speed to low and alternately beat in the buttermilk and dry ingredients, adding about 1/3 of each at a time.
Divide the batter between the prepared pans. It will be quite thick. Spread evenly.
Bake the cake for about 25 minutes. A toothpick inserted in the middle should come out clean.
Remove from the oven, and cool for at least 5 to 10 minutes before turning out onto a rack to cool completely.
Sift together the cocoa powder and powdered sugar to remove lumps. Set aside.
In a large bowl, using an electric mixer, cream together the semi-sweet chocolate and unsalted butter until smooth.
Add the pure vanilla extract, reduce speed to low, and gradually add the flour and cocoa mixture until combined.
Slowly add the buttermilk until the frosting reaches desired consistency.
Using a serrated knife, trim the top of each cake to make a flat surface.
Take the tops that you cut off, crumble into smaller pieces in a baking sheet. Drizzle melted butter over the crumbs and toss to coat.
Bake the crumbs in the oven for 20-30 minutes until browned and dried out.
Using a serrated knife, cut each cake in half. Take one layer and place on a plate, spreading 1 cup of frosting evenly on top.
Repeat with the remaining layers, then apply a thin layer of frosting over the cake.
Finish frosting the top and sides, pressing the crumbs into the sides of the cake as decoration.
