Preheat oven to 350°F. Prepare a 9-inch springform pan with parchment and grease the sides. Mix sugar and butter until light and fluffy. Add sour cream and vanilla, then egg whites, mixing well. Combine flour and baking powder, add half to batter, then add milk and water, then remaining dry ingredients. Divide batter into prepared pan and bake for 18-20 minutes, then cool.

Make the pastry cream: heat milk until boiling, pour over chopped chocolate and vanilla, whisk until smooth. In a separate bowl, beat cream cheese, sugar, and flour. Add sour cream and vanilla, then eggs. Whisk until smooth. Bake in water bath at 300°F for about 1 hour 15 minutes until just set. Cool and refrigerate.

Make chocolate ganache: heat heavy cream, pour over chocolate chips, add corn syrup and vanilla, whisk until smooth. Cool. Pipe ganache around cheesecake, then spread pastry cream on top. Refrigerate before serving.