- Preheat oven to 375°. Line 17-1/4 x 11-1/2-inch jelly roll pan with parchment paper; spray with cooking spray. In large bowl, with mixer on high speed, beat eggs 5 minutes; reduce speed to low. Add cake mix, water and oil; beat 30 seconds. Increase speed to medium; beat 1 minute. Pour batter into prepared baking pan; with rubber spatula, evenly spread batter in pan. Bake 15 minutes or until cake springs back when lightly touched.
- Dust kitchen towel with powdered sugar. While still hot, flip cake onto prepared towel; carefully peel off parchment paper. Starting from short end, roll up cake in towel; cool 1 hour.
- For filling, in large bowl, with mixer on low speed, beat 1 cup cream, 2 tablespoons powdered sugar and 1 teaspoon vanilla extract 2 minutes; increase speed to medium-high and beat 4 minutes or until stiff peaks form.
- Carefully unroll cake. Leaving 1/2-inch border, evenly spread filling over cake. Starting from short end, roll up cake with filling; refrigerate 30 minutes.
- For frosting, in large bowl, with mixer on medium speed, beat butter 2 minutes; reduce speed to low. Add cocoa powder, salt, and remaining 1/4 cup cream, 3-1/2 cups powdered sugar and 2 teaspoons vanilla extract; beat 2 minutes. Increase speed to high; beat 4 minutes or until fluffy.
- Transfer cake to cutting board; cut off 2-inch diagonal slice from 1 end of cake. Transfer cake to serving platter; spread cut side of diagonal slice with 1 tablespoon frosting and attach to side of cake. Spread frosting over cake; run fork lengthwise through frosting in curvy motions to give appearance of bark. Dust with powdered sugar.
- With scissors, cut marshmallows crosswise in half; stick 1 candy onto cut side of each marshmallow. Dust with cocoa powder.
- Decorate cake platter with candy mushrooms; garnish with chocolate shavings, sugared cranberries and sugared rosemary, if desired.
For sugared cranberries and rosemary: Line rimmed baking pan with parchment paper; spread 1 cup granulated sugar on a plate. Heat 1 cup each granulated sugar and water to a boil over medium-high heat; cook 5 minutes or until sugar dissolves. Remove from heat; cool slightly. Add cranberries to warm sugar liquid and stir to coat; with slotted spoon, transfer to plate with sugar and toss until coated. Transfer to prepared pan to dry. Repeat with rosemary sprigs.