Preheat oven to 350 F degrees.
You'll need a sheet cake pan, I used just a regular baking pan, 18 in x 12 in, which you need to spray with nonstick baking spray.
In a medium size bowl, mix together the buttermilk, eggs, vanilla, baking soda and salt together. Stir in the vinegar and set aside. In your mixer, cream the butter and sugar until fluffy. Add the flour and the buttermilk mixture and mix until mixed well. Add the cocoa and food coloring and mix for a couple more minutes, scraping the side of the bowl if necessary. Pour the batter into the baking pan and using a spatula even out the surface.
Bake for 20 minutes. You need to let the cake cool completely. After it's cooled you'll need to place a large cutting board on top of the cake pan, and then you need to flip it over, the cake will nicely fall out. I didn't have a cutting board big enough, so I just placed a parchment paper over the top of the cake pan then flipped it over. You will need to do this so that you can ice the cake.
In a saucepan add the milk and the flour and cook over medium low heat whisking continuously, until the mixture thickens. It needs to be really thick, basically until it becomes hard to whisk. At the end whisk in the vanilla extract. Let this cool.
In your mixer beat the sugar and butter together until it's light and fluffy. Since you're using granulated sugar for this you need to beat it for a long time, 5 to 10 minutes. Add the flour/milk mixture to this and continue mixing for a couple more minutes. Spread the icing over the cake evenly, then cut the cake in squares. If you prefer, add a raspberry on top of each slice.