How to Make It
Preheat oven to 400°F. Coat a 15- x 10- x 1-inch jellyroll pan with cooking spray; line with wax paper, and coat with cooking spray. Beat eggs, baking powder, and 1/4 teaspoon of the salt with a hand mixer on medium-high speed until combined, about 30 seconds. Gradually add 3/4 cup of the granulated sugar, beating until thick and light colored, about 4 minutes. Fold in flour, cocoa, and vanilla. Spread batter evenly into prepared jellyroll pan. Bake in preheated oven until cake surface springs back when gently pressed, about 12 minutes.
Sift 3 tablespoons of the powdered sugar in a 15- x 10-inch rectangle on a clean linen towel. Turn cake out onto sugar-coated towel; remove wax paper from cake. Trim crisp cake edges, if needed. Starting with the short end, carefully roll up cake and towel together, jellyroll style. Let cool completely on a wire rack, seam side down, about 1 hour.
Unroll cake; remove towel. Stir together hot brewed coffee and remaining 3 tablespoons granulated sugar in a small bowl until dissolved; brush mixture over cake. Spread with whipped cream, and reroll (without towel). Chill 30 minutes.
Stir together boiling water and instant coffee granules in a small heatproof bowl until dissolved. Melt chopped chocolate in the top of a double boiler; stir in butter, dissolved instant coffee mixture, and remaining pinch of salt until smooth. Remove from heat, and let cool until lukewarm, about 30 minutes. Stir in remaining 2 1/2 cups powdered sugar until a smooth, spreadable consistency is reached.
Spread chocolate frosting evenly over chilled cake. Score frosting with a spatula or fork tines to resemble bark of a tree. Garnish cake with candied cherries. Refrigerate at least 30 minutes or up to 3 days before serving.