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1. Preheat oven to 375°F. Line a 15x10-inch heavy jelly-roll sheet pan with parchment paper or grease and flour aluminum foil. Make sure the parchment paper or foil sticks up at least 1 inch above the sides of the pan on all sides. You’ll use the parchment paper later to lift the cake out of the pan and roll it up.
2. In a large bowl, using an electric mixer, beat eggs on high speed until very thick and lemon-colored. (If eggs are not beaten enough, the cake will be heavy and rubbery.) Gradually beat in sugar. On low speed, beat in water and vanilla.
3. In a medium bowl, whisk flour, cocoa, baking powder, and salt in a medium bowl and set aside.
4. Add dry ingredients to egg mixture gradually, beating just until smooth. 
5. Pour the batter into the prepared pan and spread evenly. Bake for 12 to 15 minutes, or until the top of the cake springs back when touched and a toothpick inserted comes out clean.
6. Remove the cake from the oven and immediately loosen the cake. Lift the cake from the pan using the parchment paper and set it on the counter.
7. While the cake is hot, use the parchment paper to slowly roll the cake up, starting at the shorter end of the cake. Set the cake aside to cool completely, for at least 30 minutes.
8. Make the filling when the cake has cooled and is ready to be filled. See below instructions for filling.
    

1. In a bowl, add heavy whipping cream, vanilla, and powdered sugar. Whip on high speed until soft peaks form.
2. Unroll the cooled cake very gently. If needed, use an offset spatula to release the cake as you unroll.
3. Spread filling evenly over the unrolled cake, being generous at the edges. Then, roll it back up without the parchment paper.
4. Wrap it in plastic wrap with the seam side down and refrigerate for at least 1 hour to firm.
5. Dust cake with additional powdered sugar, if desired.

Note: For a fancier cake, you could frost with chocolate ganache. Use a fork to draw lines down the length of the “log” to make it look more textured, like bark.