How to Make It
Prepare the Cake: Preheat oven to 325°F. Spray a 12- x 17-inch rimmed baking sheet with baking spray. Line baking sheet with parchment paper, and spray parchment paper.
Whisk together flour, ground ginger, cinnamon, baking powder, allspice, nutmeg, baking soda, and salt in a medium bowl. Set aside
Beat egg whites in a medium bowl with an electric mixer on low speed until opaque, about 5 minutes. Increase speed to high, and beat until soft peaks form, about 3 minutes. With mixer running, add granulated sugar in a slow, steady stream. Continue beating until glossy, stiff peaks form, about 2 more minutes. Set aside.
Beat egg yolks in a large bowl with mixer on high speed until thick and pale, about 2 minutes. Add molasses, brown sugar, and fresh ginger, and beat on medium speed until combined, about 1 minute. Add flour mixture, and beat on low speed just until smooth. Fold in one-third of beaten egg whites. Add remaining egg whites, and fold until just combined. Spread batter evenly into prepared pan.
Bake at 325°F until Cake is firm on top and springs back when lightly pressed, 13 to 15 minutes. Cool in pan on a wire rack 1 minute.
Dust a clean tea towel with 1/4 cup of the powdered sugar. Run a thin knife around edge of Cake to loosen from pan. Invert onto prepared tea towel. Peel off and discard parchment paper. Dust generously with remaining 1/4 cup powdered sugar. Starting with 1 short side, roll up Cake and towel, jelly-roll fashion. Let stand until cooled completely, about 1 hour.
Prepare the Filling: Beat cream cheese and butter in a medium bowl with mixer on medium-high speed until smooth and fluffy, about 4 minutes. Add powdered sugar, rum, vanilla, cinnamon, and nutmeg, and beat until smooth.
Carefully unroll Cake on a cutting board or work surface; remove tea towel, and spread Filling over cooled Cake. Starting with short end, roll up, jelly-roll fashion. Wrap tightly in plastic wrap, sealing both ends. Refrigerate at least 8 hours or up to 2 days before serving.
Prepare the Eggnog Cream: Fit electric mixer with whisk attachment. Beat cream in a chilled bowl with mixer on high speed until stiff peaks form, about 5 minutes. Fold in eggnog, rum, and nutmeg with a rubber spatula. Beat on high speed until medium peaks form, about 30 seconds. (Consistency of the cream should be midway between whipped cream and a thick sauce.) Chill until ready to serve.
Trim ends of roulade. Slice roulade using a serrated knife. Spread about 3 tablespoons Eggnog Cream on each plate, and top with a roulade slice. Serve immediately.