Lightly grease a 15- x 10- x 1-inch jellyroll pan; line with wax paper, and grease lightly.
Set aside.
Combine baking powder, 1/4 teaspoon salt, and eggs (at room temperature) in mixing bowl; beat at medium speed of an electric mixer until blended.
Gradually add sugar, beating until thick and lemon colored.
Fold in flour, cocoa, and vanilla.
Spread mixture evenly into prepared pan.
Bake at 400° for 13 minutes or until surface springs back when gently pressed.
Sift 2 to 3 tablespoons powdered sugar in a 15- x 10-inch rectangle on a linen towel.
Turn cake out on sugar; remove wax paper from cake.
Trim edges, if necessary.
Starting with short end, roll up cake and towel, jellyroll fashion.
Cool thoroughly on wire rack.
Unroll; spread with whipped cream, and reroll.
Chill.
Place chocolate in top of a double boiler; bring water to a boil.
Reduce heat to low; cook until chocolate melts.
Blend in butter, coffee, dash of salt, and boiling water, stirring until smooth.
Cool to lukewarm.
Stir in about 2 1/2 cups powdered sugar to make a spreading consistency.
Spread frosting evenly over cake.
Mark frosting with tines of a fork to resemble bark of a tree and decorate with candied cherries.
Refrigerate until serving time.