1. For the cake: Position an oven rack in the middle of the oven, and preheat to 350 degrees F. Coat two 9-inch round cake pans with cooking spray; dust with flour, and tap out the excess.
2. Whisk together the flour, granulated sugar, baking soda and salt in a medium bowl; set aside. Stir together the hot water and cocoa powder in a small bowl until smooth; let cool slightly. Stir together the milk and vinegar in another small bowl, and let stand until thickened, 3 to 5 minutes.
3. Whisk together the oil, eggs and vanilla in a large bowl until smooth. Whisk in the red food coloring and the cocoa mixture. Alternate whisking in the flour mixture in three additions and the milk in two, starting and ending with the flour, until just combined. Divide the batter evenly between the prepared pans.
4. Bake until a toothpick inserted in the center comes out clean, about 30 minutes. Let cool for 10 minutes in the pans, turn out onto cooling racks and let cool completely.
5. For the frosting: While the cake layers are cooling, beat the cream cheese, butter, vanilla and salt with an electric mixer on medium speed in a large bowl until smooth and fluffy. Gradually beat in the icing sugar until smooth. Increase the speed to high, and beat until very fluffy and there is no gritty texture left from the sugar. Beat in the milk to adjust the consistency if the frosting seems too thick.
6. To assemble: Place one cake layer bottom-side down on a cake plate. Top with 1 cup of the frosting, and spread to the edge. Top with the second layer, and frost the top and sides with the remaining frosting. If the frosting becomes too soft while assembling the cake, briefly chill the cake and frosting and then finish frosting.
7. You can bake the cake layers the day before assembling and serving the cake; wrap them in plastic wrap and then foil, and store at room temperature. Or you can bake and assemble the cake the day before, refrigerate it overnight and let stand at room temperature for a few hours before serving.
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