Preheat the oven to 350℉. Spray an 8-inch square and an 8-inch round pan with nonstick baking spray.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda and salt.
Add eggs, oil, buttermilk, food coloring, vanilla and vinegar to the flour mixture. Use an electric mixer to beat for 1 minutes, scrape sides and bottom of bowl with a rubber spatula. Continue to beat on high speed for 2 minutes.
Divide the batter evenly between the two pans. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out with just a few crumbs attached. Let cool for 10 minutes, then turn out onto racks to cool completely.
Find a cutting board or large platter that will fit the large heart cake. Place the square cake on point (one corner pointing down). Cut the round cake in half and place on the upper sides of the square to form a heart shape.