In a medium bowl, sift together the flour, cocoa, baking soda and salt. Give the bowl a whisk. Set aside.
In a separate large bowl, beat together the butter, 1/2 cup brown sugar and 1/3 cup granulated sugar until creamy.
Beat the egg, red food coloring, vinegar and vanilla into the butter mixture until combined. At this point, the dough will look very bright red but the color will soften after the dry ingredients are incorporated.
Mix the flour mixture into the butter mixture. I typically do this about 1/2 at a time, starting with the mixer on a low speed. It should be slightly difficult to incorporate all of the dry ingredients.
Beat the milk into the dough.
Cover the bowl and place in the fridge for at least 2 hours or up to 48 hours.