Preheat oven to 350 degrees F (180 C). Line 12 cupcake cups with paper liners.
Sift flour, salt and cocoa powder into a bowl and set aside.
With a hand-held electric mixer or a whisk, beat the sugar, oil, egg and vanilla together for about 2 minutes or until pale and light. Beat in the red gel coloring.
Add the flour mixture and the buttermilk to the batter alternately, stirring by hand and beginning and ending with flour.
Mix the vinegar and baking soda together in a large cup (mixture will foam up), then stir that into the batter.
Divide batter evenly among cupcake pans. Bake on center rack for 20 minutes or until cakes spring back when touched. Let cool and make icing.
Beat butter and cream cheese together. Slowly add powdered sugar and beat until fluffy. Beat in vanilla and lemon juice.