Preheat the oven to 350 degrees F and coat a 9-inch square, 2 1/4-inch-deep cake pan with cooking spray. Prepare the cake mix as directed; pour into the pan and bake until a toothpick comes out clean, 30 to 35 minutes. Cool slightly, then remove to a rack to cool completely.
Trim the top of the cake with a long serrated knife to make it level, then cut the cake into two even layers.
Keeping the layers stacked, use a small serrated knife to round out the four corners of the cake; make a notch in two opposite sides of the cake and the top to make a bread shape as shown.
Carefully transfer the top cake layer to a baking sheet and cut a 3- to 4-inch circle out of the middle.
Brush the top with the melted butter and broil until toasted.
Combine the frosting, cocoa powder and yellow food coloring in a small bowl (it should be a light golden brown). Spread about half of the frosting on the bottom cake layer.
Carefully place the top cake layer on the bottom layer (use 2 spatulas to transfer the top layer -- it will be fragile). Spread a thin layer of frosting around the edges of the cake to look like the crust.
Stir the marshmallow cream until loose, then spoon it into the hole in the cake.
Place the peach half on top of the marshmallow cream and gently press so the edges are slightly submerged. Let sit for a few minutes to let the "egg" settle. Top with grated chocolate to look like pepper.