1To make the brownie layer: Position a rack in the center of the oven and preheat the oven to 325 degrees F. Lightly grease the bottom and sides of a 9x3-inch springform pan. Cut an 18-inch square of heavy duty aluminum foil and wrap around the outside of the pan.
2In the top of a double boiler, melt the chocolate with the butter over barely simmering water, stirring occasionally. Transfer the mixture to a medium bowl and stir in the espresso powder, vanilla extract and salt. Whisk in the sugar until combined. Whisk in the eggs one at a time, until blended. Stir in the flour until no traces remain.
3Scrape the batter into the prepared pan and smooth it into an even layer. Bake the brownie for 25-30 minutes, until a toothpick inserted into the center comes out with a few moist crumbs attached to it. Place the pan on a wire rack and let cool while you make the filling. Leave the oven on.
4To Make The Latte Filling: In the bowl of an electric mixer, using the paddle attachment, beat the cream cheese at medium-low speed until creamy and smooth, about 2 minutes. Gradually add the sugar and beat until blended. Add the vanilla extract, espresso mixture, Kahlua, salt, sour cream and cornstarch and mix until well blended. At low speed, add the eggs one at a time, mixing well after each addition and scraping down the sides of the bowl with a rubber spatula as needed.
5Scrape the batter onto the brownie layer and smooth the top. Place the springform pan in a roasting pan or a large baking pan. Pour enough hot water into the roasting pan to come about 1 inch up the sides of the springform pan. Bake the cake in the water bath for 70-80 minutes, until the center is set but slightly wobbly (the cheesecake will continue to set up as it cools). Turn the heat off and crack the door, allowing the cake to cool in the oven for about 30 minutes. Remove the cake from the water bath and place on a rack and cool to room temperature.
6Refrigerate the cheesecake for at least 4 hours or overnight before serving.