Preheat the oven to 350°F (175°C). Line a jelly roll pan (15x10 inches) with parchment paper and lightly grease it.
In a bowl, whisk together flour, cocoa powder, sugar, baking powder, and salt.
In another bowl, beat eggs until frothy; mix in milk and vanilla.
Gradually add the wet ingredients to the dry ingredients, stirring gently without overmixing.
Spread batter in the pan and bake for 12-15 minutes or until a toothpick comes out clean.
Once baked, invert onto a powdered sugar-dusted towel, peel off parchment paper, roll tightly in the towel, and cool completely.
Unroll the cooled cake and spread whipped cream over the surface; roll back up without tearing.
Frost with more whipped cream, dust with powdered sugar, and decorate with meringue mushrooms if desired.
