Preheat the oven to 350ºF. Melt the chocolate in a metal bowl and set over a saucepan of boiling water or in the top of a double boiler (or melt in a microwave for 20-30 seconds). Meanwhile, place the sugar, eggs, oil, and vanilla in a bowl and mix for about one minute on low speed. In a mixing bowl, sift together the flour, baking soda, and salt. Add the dry ingredients to the egg mixture and continue to mix on low speed until well incorporated. Add the melted chocolate and mix on low speed, then add the pureed beets and food coloring. Continue to mix on low speed until all the ingredients are thoroughly combined. Divide the batter evenly between 3 greased and floured pans, and bake for 20-25 minutes, or until the cake springs back when touched, or when a toothpick comes up clean. Cool for 10 minutes in the pans, then turn the layers out onto a rack to cool completely.