Heat oven to 375°F. Grease a 15 x 10 x 1-inch jelly-roll pan. Line pan with waxed paper.
Sift together cocoa, flour, cinnamon, ginger and cardamom.
Beat egg whites with cream of tartar in bowl until frothy. Gradually beat in 1/4 cup granulated sugar; beat until stiff peaks form.
Using same beaters with mixer on high, beat yolks in second bowl with 1/2 cup sugar until thick and lemon-colored, 5 minutes. Beat in vanilla. Fold cocoa mixture into yolks. Fold yolk mixture into whites. Pour into pan, spreading evenly.
Bake in 375°F oven 14 to 16 minutes, until center springs back when gently pressed.
Sift confectioners sugar over kitchen towel. Immediately invert cake onto towel; remove pan and paper. Starting from long side, roll cake up inside towel jelly-roll style. Cool, seam-side down, on wire rack.
Unroll cake. Spread top with one-third of frosting. Gently reroll cake without towel. Cut ends off cake, using parallel diagonal cuts. Reserve ends to use as knots. Place cake, seam-side down, on serving plate. Place knot on each side of log. Spread remaining frosting over log and knots, leaving all ends exposed. For bark effect, run tines of fork over frosting. Garnish with jellied spearmint-leaf candies, halved, and small cinnamon candies for holly and chocolate mushrooms, if desired.
Dissolve instant coffee granules in boiling water; let cool. Beat confectioners sugar, butter, and coffee mixture in medium-size bowl until light and fluffy, 3 minutes.