Cake. Preheat oven to 350° F. Grease the bottoms and side of an 8 inch x 8 inch baking pan. Line the bottom and sides with parchment paper. Grease and flour the parchment paper. (If you are not using parchment paper, just grease and flour the pan, but it is much easier to release the cake using parchment paper at least on the bottom of the pan.)
In a small saucepan, bring the milk to a simmer on low heat. Place the cocoa powders in a small, heatproof bowl. Pour the simmering milk over the cocoa powder, whisk and set aside to cool.
In a small bowl, sift together the flour, baking soda, baking powder and salt. Set aside.
In a medium bowl, whisk the oil, sugar, brown sugar, eggs and vanilla until fully combined. Whisk in the cooled cocoa/milk mixture.
Stir half the flour mixture into the liquid ingredients and, before the flour is fully combined, add the sour cream. Just before all the sour cream is combined, stir in the rest of the flour, just until all the ingredients are fully combined. Do not overmix.
Spread batter into prepared pan. Bake for 43-47 minutes, just until a toothpick inserted comes out clean. Allow cake to cool in pan completely on a wire rack.
Option 1: Fluff Buttercream. With an electric mixer, beat butter with sugar and salt until light and fluffy. Add marshmallow fluff and vanilla and beat until fully combined.
Option 2: Marshmallow Crème Frosting. Fill a medium size pot about one-third with water. Place pot on a burner over medium-low heat. In the metal bowl of a stand mixer, combine all ingredients except for the vanilla. Place the bowl over the pot of water. The water should not touch the bottom of the mixing bowl. Constantly whisk the mixture until it reaches 160° F. If using pasteurized egg whites, you can go to just 140° F. (Do not stop whisking, as you do not want scrambled egg whites.) Immediately remove the mixing bowl from the heat and insert it onto the mixer with the whisk attachment. Whisk at medium-high speed until stiff peaks form. (Start mixing immediately. The mixture will cool as it is whipped.) Add vanilla and whisk an additional 30 seconds until fully incorporated.
Assembly. Remove the cake from the pan. With a serrated knife, cut off thin slices (no more than ¼ of an inch thick) from each of the four sides of the cake, each the full length of the cake. Crumble the thin slices into a bowl and set aside. Spread the fluff buttercream or the marshmallow crème frosting on the top of the cake. Sprinkle with the cake crumbs.