Heat the oven to 350°F. Grease two 8 inch round pans and line the bottoms with parchment.
Whisk the flour, cornstarch, cocoa, baking soda, baking powder, and salt in a bowl. Set aside.
Beat the butter, oil, and sugar together 2 to 4 minutes until pale and fluffy. Add the eggs one at a time, beating well after each. Mix in the vanilla, vinegar, and red coloring until the batter looks evenly red.
Add the dry mix in three parts, alternating with the buttermilk, starting and ending with dry. Mix gently and stop as soon as no flour streaks remain.
Divide between the pans and bake 25 to 28 minutes, until a toothpick comes out clean. Cool in the pans 10 minutes, then turn out onto a rack to cool fully.
For the ermine frosting, whisk the sugar, flour, and salt in a saucepan, then slowly whisk in the milk until smooth. Cook over medium heat, stirring constantly, until thick like pudding. Cool completely to room temperature.
Beat the butter until fluffy, then add the cooled milk mixture a spoon at a time, beating until silky. Beat in the vanilla and salt.
Level the cake layers if domed. Spread frosting between the layers, then cover the top and sides. Decorate with red crumbs if you like. Chill 30 minutes before slicing for clean cuts.