To begin, preheat your oven to 350 degrees. Lightly grease a bar pan. Line the bottom with parchment paper and grease the paper. Set aside.
In a bowl, sift together the flour, baking cocoa, baking powder, and salt. Set aside. In a mixing bowl, beat together the eggs and sugar. Add in the vanilla. Mix in the dry ingredients until well combined. Spread the batter evenly over the bottom of the bar pan. Bake for 13-14 minutes or until it tests done in the center with a toothpick. Carefully remove the cake from the pan and let it cool on a rack.
When it is cool, evenly spread the Nutella all over the surface of the cake.
In your clean mixing bowl, beat the butter and cream cheese until it is fluffy. Mix in the vanilla and powdered sugar and beat it until it is smooth. Put the filling in dollops all over the cake and then spread it evenly. Roll the cake up from the end and place it on a platter, seam side down. Cover it with plastic wrap and chill it at least 2 hours in the fridge. (Overnight is perfect.)
The next day, give the roll a light dusting with powdered sugar. Cut the roll into slices with a sharp knife and line them up on a serving platter. Give the slices another light dusting of powdered sugar if you like.