Prepare the ingredients. Separate the egg yolks and whites. Beat the egg whites with 1/4 cup powdered sugar on high speed until stiff peaks form. In another bowl, beat the yolks with the remaining 1/4 cup powdered sugar, oil, vanilla, and salt.

Beat on high speed until the mixture is light and fluffy. Sift together the flour, cocoa powder, and baking powder, then gently fold into the yolk mixture on low speed until just combined.

Gradually fold the egg whites into the yolk mixture, then add the cream of tartar and mix until fully incorporated.

Pour the batter into a greased and parchment-lined 12x17 inch baking pan, spreading it evenly. Bake at 350°F for 12 to 15 minutes.

Lay out a clean kitchen towel and generously dust it with cocoa powder. Invert the cake onto the towel and roll it up with the towel. Let the rolled cake cool completely.

To make the filling, beat the heavy cream and vanilla on medium speed until stiff peaks form. Soften the sour cream and fold it into the whipped cream to make a smooth filling. Carefully unroll the cooled cake, spread the cream filling evenly over the surface, and use the towel to roll the cake back up into a log shape. Place the cake on a serving tray.

Decorate the cake as desired. Melt the chocolate with 3 tablespoons of milk, let it cool, then fold it into some of the cream to create a log-like color. Decorate as you like.

Your Yule log cake is ready to serve.