Line a 13x9 inch swiss roll pan with parchment paper.
Preheat oven at 190C.
Beat the egg whites with the cream of tartar and sugar till stiff peaks. I use medium speed (on my Kitchenaid, that's number 6 out of 8) for 4 minutes.
Meanwhile mix the egg yolks, flours, coffee+chocolate mixture, brown sugar, coloring and buttermilk with a balloon whisk in a separate bowl. Just make sure it is smooth, never overmix
Add the vinegar to the egg yolk mixture..
Mix the whites with the egg yolk mixture, 1/3 at a time.
Pour this into the swiss roll pan. It is important to level the top for easier rolling later.
Bake for 10 minutes at a high to medium rack.