For filling: Bring milk to boil in medium saucepan over medium heat. Whisk yolks and sugar in small bowl until well blended; whisk in flour. Whisk hot milk into egg mixture. Return to same saucepan. Whisk over medium heat until custard thickens and boils, about 1 1/2 minutes. Transfer to processor; cool to room temperature, about 1 hour. Add marzipan; process until smooth, about 1 minute. Blend in butter 1 piece at a time, then both extracts. Cover and refrigerate filling at least 4 hours or up to 2 days.
For cake: Preheat oven to 350°F. Butter 11x10x1-inch jelly roll pan. Line bottom with parchment paper; butter and flour parchment. Whisk eggs, 3/4 cup sugar, vanilla and salt in large metal bowl. Set bowl over pot of simmering water. Whisk until just warm, about 2 minutes. Remove bowl from over water. Using electric mixer, beat mixture until tripled in volume, about 6 minutes. Sift flour over; fold in until just combined. Gently fold in butter (do not overmix, or batter will deflate). Spread batter in prepared pan. Bake until tester inserted into center comes out clean, about 20 minutes. Sift powdered sugar over towel. Run knife around edge of pan to loosen cake. Turn hot cake out onto towel. Peel off parchment. Starting at 1 long side and using towel as aid, gently roll up cake jelly-roll style. Cool. Unroll cake. Spread filling over. Reroll cake; place cake, seam side down, on platter. Refrigerate cake while preparing frosting.
For frosting: Bring cream and butter to boil in medium saucepan over medium-high heat, stirring to melt butter. Remove from heat. Add both chocolates; whisk until melted. Transfer to medium bowl. Let cool at room temperature until thick enough to spread, about 1 hour.
Starting 1 inch in from each end of cake, cut on diagonal to remove one 3-inch-long piece of cake from each end. Attach 1 cake piece at sides of cake near each end. Spread frosting over top and sides of cake and pieces. Using tines of fork, draw concentric circles on cake ends to resemble tree rings. Draw fork along length of cake to form bark design. Garnish with Marzipan Mushrooms.