Cake: Grease two 8-inch (1.2 L) round cake pans; line bottoms with parchment paper. Set aside.
In large bowl, beat butter with sugar until fluffy; beat in eggs, 1 at a time. Beat in vanilla and molasses. In separate bowl, whisk together flour, cocoa powder, baking powder, baking soda and salt; stir into butter mixture, alternating with buttermilk, making 3 additions of flour mixture and 2 of buttermilk. Scrape into prepared pans, smoothing tops.
Bake in 350°F oven until cake tester inserted in centres comes out clean, about 35 minutes. Let cool completely in pans. (Make-ahead: Wrap each in plastic wrap; refrigerate for up to 24 hours or overwrap in foil and freeze for up to 2 weeks.)
Pomegranate Glaze: While cake is cooling, in small saucepan, bring sugar, molasses, rosemary and 2 tbsp water to boil, stirring, over medium-high heat. Reduce heat and simmer until reduced by half, about 5 minutes. Discard rosemary. Let cool completely.
Mascarpone Icing: Meanwhile, in bowl, beat together mascarpone, icing sugar and vanilla until smooth. In separate bowl, beat cream until stiff peaks form; fold into mascarpone mixture.
Assembly: Invert cakes from pans; peel off parchment paper. Place 1 cake layer on cake plate; brush with half of the glaze. Sprinkle with 1/4 cup of the pomegranate seeds.
Spoon icing into piping bag fitted with large plain tip; pipe about half of the icing into mounds over top of cake layer. Top with second cake layer; brush with remaining glaze. Pipe remaining icing into mounds over top of cake. Refrigerate until icing is set, about 25 minutes. (Make-ahead: Refrigerate for up to 2 hours.)
Sprinkle cake with remaining pomegranate seeds, rosemary and decorating sugar.
Makes 12 to 14 servings.