1. In a bowl, beat egg yolks, sugar and vanilla on high until thick and lemon-colored, about 5 minutes. Reduce speed to low; beat in chocolate and water until blended. Add flour and baking soda; mix well.
2. In another bowl, beat egg whites on high until stiff peaks form. Fold in the chocolate mixture. Spread evenly in a greased and floured 15-in. x 10-in. x 1-in. baking pan. Bake at 375° for 15 minutes or until a toothpick inserted near the center comes out clean. Cool in pan on a wire rack for 5 minutes.
3. Meanwhile, sprinkle a kitchen towel with confectioners' sugar. Invert cake onto the towel. Starting from a short end, roll up cake in the towel. Place seam side down on a wire rack to cool completely. Unroll cake; spread coconut-pecan frosting to within 1 in. of edges; roll up cake without the towel. Cut and reserve a narrow diagonal slice from each end of the cake. Reserve 2 tablespoons chocolate frosting for mushrooms. Attach diagonal cake slices with a little of the frosting to opposite sides of cake with remaining frosting. With the tines of a fork, make strokes in the frosting to resemble bark on a tree trunk.
4. For the mushrooms, half marshmallows widthwise. Flatten half of each marshmallow for cap of mushroom; roll other half between palms of hands for stem. Attach caps to stems with the reserved frosting. Dust tops of mushrooms with cocoa; place around the log. Yield: 8 servings.