Beat eggs and sugar until fluffy with electric mixer. Add the baking powder. Add cocoa, sift it if you want to avoid lumps. (Aiming means that you pour it in a sieve to shake the cocoa through.) Add the last potato flour and whisk until smooth. Spread the batter in a roasting pan with parchment paper, 30 x 40 cm. Bake in oven at 250 degrees for 5 minutes. If you have a convection function on your oven bake it at the center at 200 degrees for 8 minutes set to hot air. I think it's easier to get good results in the convection oven. If you use regular oven so please note that you really need to help "guard" roll cake last minute so that it does not burn. Turn out immediately roll up the cake on the sugared parchment paper. Peel the parchment paper as a cake baked on. (Brush with cold water on the back of the paper if it is not to come off.) Allow to cool roller cake. make the filling. Let the butter come to room warm. Cream the butter and icing sugar until smooth, often with electric mixer. (Feel happy and if you think it tastes too much butter, mix in more sugar.) Add the vanilla sugar and finally the egg yolk. Spread the filling over the cake and roll up. Place rolled cake cool in the refrigerator at least 20 minutes before carving it. Keep rolling the cake in the refrigerator.