Cream together butter, cream cheese and sugar in a stand mixer on medium low for about 1 minute until smooth. Add egg yolk, vanilla, red food coloring and mix until combined on medium speed. Scrape bottom and sides and slowly add flour, cocoa, salt and corn starch and mix on medium low until a stiff dough forms. (about 1 minute) Wrap dough in plastic wrap and chill for 1 hour.