Sift 1/2 cup powdered sugar, cocoa powder,and espresso powder into small bowl. Usingelectric mixer, beat cream and vanilla inlarge bowl until soft peaks form.
Add cocoamixture and beat until stiff peaks form.
Spread 1 side of 1 chocolate waferwith 1 rounded teaspoonful mocha cream;top with another wafer. Continue layeringwafers and mocha cream for stack of 5cookies.
Place stack on its side on longplatter. Repeat making stacks withremaining wafers and some of mochacream; form log on platter by attachingstacks with mocha cream. Using offsetspatula or rubber spatula, spread remainingmocha cream over outside of log to coat.Cover; chill at least 2 hours. DO AHEAD: Canbe made 1 day ahead. Keep chilled.
Place platter with yule log on worksurface. Using fork, gently pull tines of forkalong length of frosting on log to create designresembling tree bark. Sift powdered sugarover log to resemble snow. Arrange meringuecookies or Meringue Mushrooms along sidesof log.