Given the task definition and input, reply with output. In this task, you will be presented with the directions of a recipe separated by "," and have to fill in the "___" which is a step that is missing from the recipe.

Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 9 inch pans.,______,In a large bowl, mix flour, sugar, and baking soda. Add chocolate mixture, buttermilk, eggs and vanilla. Beat until smooth. Pour batter into two 9 inch round pans.,Bake at 350 degrees F (175 degrees C) for 30 to 35 minutes, or until a toothpick inserted into cake comes out clean.
In a medium saucepan, heat butter, yogurt, chocolate and cola until chocolate is completely melted.