Mix flour, sugar, and salt in a large bowl. Set aside.
Heat butter, Coca-Cola, and cocoa powder in a saucepan over medium heat. Stir as butter melts. Bring to a boil then remove from heat.
Slowly pour the butter/chocolate mixture over the dry ingredients. Stir.
Mix eggs, milk, vinegar, baking soda and vanilla together in a small bowl. Add to the large bowl and mix until combined.
Pour batter into a greased 9x13" baking pan.
Bake for 25-30 minutes.
Frosting Instructions
Heat butter, Coke, and cocoa in a saucepan over medium heat stirring, until butter melts. Remove from heat. Pour over powdered sugar. Add vanilla and beat until smooth.
Pour frosting over the hot cake. Smooth with rubber spault.