Preheat the oven to 350F degrees. Line a 10x15 inch jelly roll pan with parchment paper and grease the parchment.
In a medium bowl whisk together the flour, 1/4 cup cocoa, baking powder and salt.
In a large bowl beat the eggs and sugar until they're pale at frothy (at least 2 minutes).
Then beat in the vanilla extract and oil.
With the mixer on low, carefully mix the dry ingredients into the eggs. Turn off the mixer and scrape down the sides of the bowl as needed.
Pour the batter into the prepared pan and bake for 11-13 minutes or until the top is fully set and springs back when you touch it.
As the cake is baking, place a large piece of parchment paper (or tea towel) on the counter (ensure that it's longer than the jelly roll pan). Dust with 3 tablespoons cocoa powder.
Invert the hot cake onto the parchment paper on the counter. Be careful here, because the pan is hot.
Remove the pan and peel off the parchment paper that lined the bottom. Starting from the short end, roll the cake & parchment paper dusted with cocoa. Cool the cake fully as it's rolled up.
Marshmallow Frosting
Beat the butter until fluffy. Then mix in the marshmallow fluff.
Beat in the powdered sugar, then mix in the cream 1 tablespoon at a time until its thick. Add in a little more powdered sugar if it's too thin.
Unroll the cooled cake and frost leaving about 1/2 inch around each side. Make sure the cake is 100% cooled before frosting.
Roll the cake back up (without the parchment), leaving the seam side down. Place in the fridge for 30 minutes to let the cake firm up.
Ganache
Add the chocolate, cream, & butter to a heat-proof bowl.
Microwave on medium heat for 30-second intervals, stirring between each. When the chocolate is almost melted, whisk until smooth.
Place the cake on a wire rack with a cookie sheet or piece of parchment paper underneath. Pour the ganache over top.
Place the cake in the fridge to firm up before serving. Optionally dust with powdered sugar and decorate with fruit before serving.