Preheat the oven to 350˚F and line a 9×13 inch pan with parchment paper.
In a mixing bowl, combine gluten-free flour, oat flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, use an electric mixer to cream together butter and sugar until fluffy (about 1 minute). Add vanilla, applesauce, and maple syrup, and continue mixing.
Stir freshly brewed espresso into oat milk.
Alternate adding the dry mixture and the oat milk/espresso to the wet ingredients, mixing on medium-low until fully incorporated. Add melted chocolate and continue mixing until incorporated.
Pour the batter into the pan and smooth it out with a spatula. Bake for 28-30 minutes, or until a toothpick comes out clean. Remove from the oven and let it cool completely before frosting.
To make vegan chocolate buttercream, melt chocolate in 30-second intervals in the microwave, stirring well between each interval, until fully melted. Set aside and let it cool until it’s cool to the touch.
Beat butter and powdered sugar on medium-high until light and fluffy – about 3 minutes. Add cooled chocolate, vanilla, and salt, and continue mixing on medium-high for another 2 minutes.
Frost the cooled cake and top it with chocolate shavings or sprinkles. Slice and enjoy!