Heat oven to 375.
Grease a 15x10x1 baking or jelly roll pan.
Line it with a waxed paper rectangle cut 1/2 inch smaller than the pan (14×9).
Grease the paper.
Sift flour, cocoa, baking powder and salt into a bowl or onto another sheet of waxed paper.
Beat eggs until thick and creamy.
Beat in sugar, 1 tbsp a time until mixture is thick.
Stir in water and vanilla.
Fold in flour mixture.
Spread batter evenly into prepared pan.
Bake 12-15 minutes until centre springs back when pressed lightly with a fingertip.
While the cake is baking, dust a large clean tea towel generously with icing sugar.
Remove baked cake from oven and immediately EITHER cut around the warm cake, 1/4-inch from the edges of the pan with a sharp knife, "discarding" scraps, OR loosen the edges of the cake from the sides of the pan with a knife.
Invert the pan onto the clean towel dusted generously with icing sugar.
Peel off waxed paper.
Gently roll the wider length of cake jelly roll style in towel. You want a long slim roll, not a short, thick one.
Cool rolled and wrapped cake completely on a wire rack. [You can store the cake like this for up to one day or re-roll the cake in wax paper, wrap wax-paper rolled cake in foil and freeze for 2-3 weeks.]
While the cake is cooling, prepare the Coffee Cream and Chocolate Buttercream Frosting. You may also prepare the Make-Ahead Mushrooms, if desired, if you haven't done so already.
Spread with Coffee Cream filling, below.
Reroll the cake.
Cut a 1/2in-or-thicker slice from the ends of the cake roll to use as knots on the cake. If you just want a festive jelly roll, move on to step 25.
Place the larger rolled cake on a serving plate.
Use Chocolate Butter Frosting [below] to attach the "knots" to the cake roll.
[Optional] Take a small bit of icing and “dirty ice” the cake with a thin layer of icing and let cake sit in the refrigerator for a while.
Frost cake roll with remaining Chocolate Butter Frosting. You do not have to ice the open ends and open knots of the cake. This is a bit tricky, but can be done with a clean icing spreader.
If desired, use the tines of a fork or an icing spatula to make the icing look like realistic “bark.”
If desired, sprinkle the ends of the roll and/or the knots/limbs with chopped pistachios, or flaked or toasted coconut.
If desired, dust with icing sugar.
If desired, decorate with Meringue Mushrooms [below]
Chill until 10-20 minutes before serving time.