Line two 7-inch cake pans with a parchment paper and spray with cooking spray.
Preheat the oven to 350°F (175°C).
In a mixing bowl with paddle attachment, combine flour, sugar, cocoa powder, baking soda and salt. Mix on low speed for 30 seconds, or until everything is well combined.
Add milk, eggs, oil and butter. Mix the batter just until smooth, about 15 seconds.
Stir in vinegar and mix for another 10 seconds.
Divide the batter into the prepared baking pans. Bake for 35 minutes, or until inserted toothpick comes out clean.
Let cakes cool completely in the pans set on wire rack. Then carefully run a knife around the edges and invert.
Wrap each cake in a plastic wrap and refrigerate until ready to use, preferably overnight and up to 2 days. Chilled cakes are sturdier and slice much easier, but you can definitely frost the cake as soon as they cooled completely.