Preheat an oven to 350 degrees F (175 degrees C). Spray 2, 9-inch springform pans with cooking spray.
Sift flour, salt, baking soda, and cocoa powder into a large bowl. Whisk in oil, 2 eggs, vanilla extract, vinegar, white sugar, and buttermilk. When well blended, whisk in food coloring and water. Pour half of the batter into each prepared pan.
Bake cakes in preheated oven until cake toothpick inserted in the center comes out clean; about 30 minutes.
While the cake is cooling, use an electric mixer to beat the cream cheese in a large bowl until smooth. Mix in the sugar and cornstarch; beat until creamy. Add the 2 remaining eggs, one at a time, beating well after each addition. Blend in vanilla. Use a hand whisk to mix in the heavy cream until well blended. Pour half of the batter over each cake layer.
Bake cheesecakes in preheated oven until set and beginning to brown, 30 to 45 minutes. Cool on a wire rack for 1 hour. Cover and refrigerate until completely cool, about 1 additional hour.
