Heat oven to 350 degrees and line a 10x15x1 rimmed sheet pan with parchment paper. Do not grease the pan. Set aside.
Whisk dry cake mix with the cocoa powder until well combined. Prepare the cake mix according to package instructions.
Pour into prepared pan and bake for 20 minutes, until the edges start to crack.
Note: The cake will rise up in the pan, but should not flow over.
Loosen cake from the edges of the pan while it's still warm. It can be a little sticky, like an angel food cake, but it will come away easily.
Spread out a clean lint free kitchen towel and sprinkle 1/4 cup powder sugar evenly all over the kitchen towel.
Flip the warm cake out onto the sugar dusted kitchen towel. Peel the parchment paper off the back side of the cake.
Start rolling the cake up with the kitchen towel, starting at the short end of the cake. With the towel rolling with the cake inside.
Let the cake cool completely at room temperature for a couple hours or pop it into the freezer up to 20 minutes if in a hurry.
Note: It's important to let the cake cool completely before adding the cake filling.
Gently unroll the cooled cake (leave it laying on the towel, if you'd like) and spread the cool whip over the top of the cake.
Then spread the mini marshmallows and semi-sweet chocolate chips.
Carefully roll the cake bake up (without the towel).
Place the cake on a serving plate with seams facing down.
Sprinkle the top with powder sugar and cocoa powder and add some fresh raspberries if desired.
Serve right away or you can keep it chilled in the fridge for a few hours before serving.