1. For brownie layer, preheat oven to 350ºF (180ºC).
2. Grease a 9-inch (23-cm) springform cake pan and line bottom and sides with parchment paper. Melt butter and pour into a large bowl. Sift cocoa into butter and stir in sugar. Add eggs to mixture, blending well after each addition. Stir in vanilla. In a separate bowl, combine flour, baking powder and salt (do not sift). Add to cocoa mixture and blend gently. Pour into pan and bake for 35 minutes, until firm. Cool completely before filling.
3. For cheesecake layer, lay 8 Oreo cookies in a circle on cooled brownie. Place chopped chocolate in a bowl on a pot of gently simmering water (be sure bowl does not touch the water) and stir to melt. Remove from heat. Beat cream cheese until fluffy with electric beaters or in a stand mixer with the whisk attachment on high speed. Slowly add sugar while mixing and beat in vanilla. Pour in whipping cream and whip into cream cheese on high speed, until mixture becomes firm and holds a peak. Scrape chocolate into mixture and blend quickly. Scrape filling onto brownie base and spread evenly. Chill cake for at least 2 hours before slicing.
4. To garnish, cut remaining 6 Oreo cookies in half. Remove springform ring and parchment paper and arrange cookie halves upright on top of cake for garnish. Serve with Cinnamon Hot Fudge Sauce (recipe follows).
Chocolate desserts will only be as good as the quality of chocolate you use. Invest in a high grade of chocolate (58% cocoa solids or higher) and you'll be thrilled with the results.
Cinnamon Hot Fudge Sauce:.
1. In a small, heavy saucepan, heat evaporated milk and sugar until sugar is dissolved. Stir in chocolate, butter and corn syrup and bring to a boil. Boil for 8 minutes, stirring often, until sauce has thickened. Remove from heat and stir in vanilla, cinnamon and salt. Serve fudge sauce warm or chilled.