For brownie layer, preheat oven to 350 degrees F. Grease a 10-inch springform cake pan and line bottom and sides with parchment paper.
Melt butter and pour into a larger 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. Stir in white chocolate chips. Pour into pan and bake for 35 minutes, until firm. Cool completely before filling.
For cheesecake layer, place chopped chocolate in a bowl of 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 lest 2 hours before slicing. To garnish, remove springform pan and drizzle top with white chocolate.