Make cake: Preheat oven to 350 degrees F. and butter and flour an 8-by 2-inch round cake pan.
In a large bowl with an electric mixer beat together butter and sugar until light and fluffy. Add remaining cake ingredients and beat on low speed until combined. Beat batter on high speed until pale yellow, about 3 minutes.
Pour batter into prepared pan and bake in middle of oven 40 minutes, or until a tester comes out with a few crumbs adhering.
Make rosemary syrup while cake is baking: In a small saucepan simmer all syrup ingredients except vanilla 10 minutes. Remove pan from heat and stir in vanilla. Cool syrup 30 minutes and strain through a sieve into a 2-cup measure.
Cool cake in pan on a rack 10 minutes. Invert cake onto hand and return, right side up, to rack. While cake is still warm, gradually brush 1/3 cup syrup over it, allowing syrup to soak in before adding more. Chill remaining syrup in a small pitcher, covered. Syrup-soaked cake may be made 1 day ahead and kept wrapped in plastic wrap at room temperature.
Serve cake, cut into wedges, with whipped cream, blackberries, and remaining rosemary syrup.
Recipe courtesy of Gourmet Magazine