Preheat oven to 325°F. Butter and flour an 8-inch springform pan.
Make torte base: In a medium bowl with electric mixer at medium speed, cream butter and sugar until fluffy. Beat in egg and vanilla. Sift together flour, cocoa powder, baking soda and salt onto a sheet of waxed paper. Add half of dry ingredients to butter mixture. Beat until incorporated, then beat in buttermilk, followed by remaining flour mixture, beating well after each addition and scraping down sides of bowl as needed. Pour into prepared pan and bake until a toothpick inserted in center comes out clean, about 30 minutes. Let cool completely on a wire rack. Allow ice cream to soften.
To assemble torte, run a knife around pan rim, then remove rim. Leave cake base on pan bottom. Using a serrated knife, trim off mounded top of cake. Line inside of pan rim with parchment or waxed paper strips, then close it tightly around cake. Spread softened vanilla ice cream evenly over torte base; place in freezer until slightly firm, 1 hour, and allow sorbet to soften.
Spread sorbet evenly over ice cream. Cover torte with plastic wrap, return to freezer and freeze until firm, at least 8 hours or up to 2 days. About 20 minutes before serving, make glaze: In a small saucepan over medium heat, bring cream and corn syrup to a simmer and then remove from heat. Add chocolate and stir until melted and smooth. Remove springform rim from torte and peel off paper. Working quickly, spread glaze over sorbet and top decoratively with raspberries. Return torte to freezer for 15 minutes. Cut into wedges and serve immediately.