Preheat the oven to 350°F (175°C). Grease two 9-inch round cake pans.

In a large bowl, beat together sugar and shortening until light and fluffy. Add eggs one at a time, mixing well after each.

In a small bowl, combine red food coloring and cocoa powder to create a paste; add this to the creamed mixture.

In another bowl, mix buttermilk, salt, and vanilla extract. Gradually add this mixture to the flour, alternating with the buttermilk mixture, until just combined.

In a separate bowl, mix vinegar and baking soda; gently fold this into the cake batter.

Pour the batter evenly into the prepared pans and bake for about 30 minutes, or until a toothpick inserted in the center comes out clean.

Cool the cakes on a wire rack for 5 minutes, then invert onto a serving plate to cool completely.

For the icing, heat milk and flour in a saucepan over low heat, stirring constantly until thick. Let it cool completely.

Beat sugar, butter, and vanilla in a large bowl until light and fluffy. Gradually add the cooled flour mixture and beat until smooth.

Frost the cooled cake layers and enjoy!