Hatsune Miku Messenger Bag Cake

Made a cake mix, added a little chocolate syrup to it (to give it a little chocolate flavor), added marshmallows (but they melted completely, so that didn't work out at all), so instead I made a buttercream with peanut butter, nutella, chocolate chips, chocolate syrup, and marshmallows, cut the cake in half, filled it with that buttercream mix, stuck the two halves together, put it up sideways, covered it in a nutella/peanut butter buttercream (no chocolate chips and marshmallows), covered it in fondant, and had gumpaste cutouts of Hatsune Miku, the logo, and accents, that I let dry and stuck to the front