In a medium bowl. Mix the potato flour, cocoa and baking powder together. Set aside.Using a large mixing bowl, beat the eggs until fluffy. They should be light yellow and airy. Add the sugar and beat until peaks form. Approx 5 minutesSlowly add the dry ingredients to the egg mixture and carefully fold into egg mixture.Pour the mixture onto the parchment paper on your prepared pan. Spread slightly. The batter will not fill the pan.Bake in a 425 degree oven for 5-7minutes. Spread your tea towel on the counter and sprinkle it with powdered sugar. Quickly turn over your cake pan and dump the cake onto the towel. Peel back the parchment paper. Starting with the long side. Roll the cake carefully up with the towel. Set aside until fully cooled.
Filling
Mix the room temperature butter and the powdered sugar until fluffy. Separate the egg yolk from the egg white. Add the egg yolk to the butter mixture. Beat well. Add the vanilla beat well again. * NOTE This recipe calls for raw egg yolk* This can be omitted if you are not comfortable using raw egg.
Putting on the frosting
Carefully unroll the cooled cake. Carefully spread the buttercream filling on top of the cake. Roll up. Cut the roll in half and wrap in plastic wrap. Store cake in the refrigerator until ready to serve. Slice the cake into 3/4" slices. Serve and enjoy. Store in the refrigerator for 1 week. Freeze up to 6 months.
Notes
The cake will last up to 1 week in the refrigerator and up to 3 months in the freezer.