Preheat oven to 350°F. Grease and line a 9 x 13 pan with parchment paper.
In a large bowl, whisk together sugar, cocoa powder, gluten free flour, baking soda, baking powder, and salt. In a separate bowl, whisk oil, eggs, and buttermilk.
Pour the wet ingredients into the dry, whisking until no lumps remain. Pour in brewed coffee, mixing until just combined.
Pour batter into prepared pans and bake for roughly 30 minutes, or until a knife inserted in the center of the cake comes out clean. Cool completely on a wire rack before frosting.
To make the frosting, place all frosting ingredients in mixing bowl. Beat slowly at first to incorporate and then beat on high speed for 1 minute. Scrape down the bowl and repeat until the frosting has lightened slightly and is fluffy.
Frost cake, spreading the frosting evenly over the whole surface, using the back of a spoon to create big swirls. Top with sprinkles if desired and enjoy!
