In a small bowl, whisk together flour, sifted cocoa, baking powder, and salt. Set aside
In large mixing bowl beat the eggs with whisk attachment on medium-high speed until thick and pale in color about 5 minutes.
Add sugar and vanilla, beat another 3 minutes or until well blended and mixture leaves a trail in the bowl.
Fold dry ingredients into the batter. Batter will start out streaky, continuing folding until mixture is completely blended.
In a small bowl or measuring cup, whisk together the buttermilk and oil. Fold the buttermilk mixture into the cake batter just until blended.
Pour batter (batter is very runny) into prepared pan and evenly spread the batter to all four corners.
Bake about 10 minutes or until center of cake springs back when gently pressed.
Let cake cool 5 minutes in pan. Then use a small knife to loosen the edges of the cake.
Using the parchment under the cake roll the cake up from the long side, not the short side.
Place rolled up cake, seam side down, onto a cooling rack and cool completely.
Filling
In a mixing bowl, whip heavy cream until soft peaks form.
In separate mixing bowl, beat together cream cheese, powdered sugar, and vanilla until smooth. Fold the whipped cream into the cream cheese mixture a little bit at a time until blended.
Unroll cake. The inside edge may stay slightly rolled, this is ok.
Spread filling onto the cake leaving ½ inch border on the outer edges as filling will be pressed to the edges as the cake is rolled back up. If inner edge keeps rolling back up do your best to spread filling under the fold.
Roll cake back up without the parchment paper. Make sure it is set seam side down. Chill completely, about 1-2 hours before decorating.