Preheat the oven to 350°F (175°C). Line a 10×15 inch jelly roll pan with parchment paper and lightly grease it.
In a large bowl, beat the egg yolks with 1/2 cup of granulated sugar until the mixture is pale and thick, about 3-4 minutes. Stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, and salt.
Gently fold the dry ingredients into the egg yolk mixture until just combined.
In a clean bowl, beat the egg whites with cream of tartar until soft peaks form. Gradually add the remaining 1/4 cup granulated sugar and continue beating until stiff peaks form.
Carefully fold the egg whites into the chocolate batter in three additions, being careful not to deflate the mixture.
Pour the batter evenly into the prepared pan and spread it out gently.
Bake for 10-12 minutes or until the cake springs back when lightly touched.
While the cake is baking, lay a clean kitchen towel on a flat surface and dust it generously with powdered sugar.
Once the cake is done, immediately invert it onto the prepared towel. Carefully peel off the parchment paper.
Starting from the short edge, roll the cake and towel together into a tight spiral. Let it cool completely on a wire rack.
For the filling, melt the chocolate chips and butter together in a microwave-safe bowl in 20-second intervals, stirring until smooth. Let cool slightly.
In a chilled bowl, whip the heavy cream with powdered sugar until soft peaks form. Gently fold the melted chocolate into the whipped cream until well combined.
Unroll the cooled cake gently and spread the chocolate cream filling evenly over the surface.
Roll the cake back up without the towel, wrap it in plastic wrap, and refrigerate for at least 1 hour before serving.
Slice and serve chilled.