Preheat your oven to 350°F and line a muffin tin with liners. Also, give a 9-inch cake pan a quick spray with nonstick cooking spray and set it aside.
In a stand mixer, mix together granulated sugar, oil, melted butter, eggs, vanilla, and buttermilk on medium-low for about 1 minute.
Add white vinegar, flour, cornstarch, baking soda, and salt. Mix on low until smooth.
Take out 1 cup of batter, dye it red. Add green food dye to the remaining batter. Cover with plastic wrap until needed.
Pour the green batter into the prepared cake pan and bake for 15 minutes. Allow it to cool to room temperature and then freeze it for at least 30 minutes.
Use a ½ inch heart cookie cutter to punch out 12 hearts from the red cake. Fill each muffin tin ⅓ full, press a heart in each one, cover with batter, and bake for 20 minutes or until done. Let them cool completely.