For the red velvet cake: Position an oven rack in the center of the oven and preheat the oven to 350 degrees F. Line two 8-inch round cake pans with parchment paper, then grease with cooking spray and dust with flour.
Whisk together the flour, cocoa powder and salt in a medium bowl and set aside. Combine the buttermilk, vanilla and food coloring in a liquid measuring cup and set aside.
Cream the butter and sugar together in a stand mixer fitted with a paddle attachment until the butter lightens in color. Add the eggs and beat until well combined, 90 seconds to 2 minutes. Alternate adding the dry and wet ingredients to the batter while mixing.
Dissolve the baking soda in the vinegar in a small bowl, then fold into the batter.
Divide the batter evenly between the cake pans and bake until an instant-read thermometer inserted in the center of the cakes reads 195 to 200 degrees F, 30 to 35 minutes. Cool cakes in the pans for 15 minutes, then turn them out onto a cooling rack and cover lightly with a linen towel. Cool completely.
For the ermine frosting: Meanwhile, heat the milk, sugar and flour in a medium saucepan over medium-low heat, whisking continuously, until the mixture becomes thick like pudding, about 2 minutes. Remove from the heat and let cool completely to room temperature.
Cream the butter, vanilla and salt together in a stand mixer fitted with a paddle attachment until smooth, about 1 minute. Add the cooled milk mixture and beat until the frosting lightens in color and becomes fluffy, at least 1 minute.
Place one cake layer on a cake board. Using a spoon or offset spatula, spread frosting generously over the top of the cake in a smooth layer. Invert the second cake layer onto the first so that the flat side is facing up, then spread the remaining frosting over the top of the cake, creating swirls in the frosting with the spoon or spatula. Transfer the cake to a cake stand and decorate the top of the cake and the stand with edible flowers and flower petals.