cereal milk cake with lucky charms filling - Pass the Cookies

Ingredients

• 1 cup (2 sticks) butter, at room temperature
• 1 1/3 cups sugar
• 4 egg whites
• 1 tablespoon clear imitation vanilla
• 6 tablespoons vegetable oil
• 2 ½ cups flour
• 2 ¼ teaspoons baking powder
• ¾ teaspoon salt
• 1 cup cereal milk (recipe below)
• Marshmallows from cereal (like Lucky Charms), for assembly
• 1 ½ cups milk
• 1 ½ cups frosted cornflake cereal
• 2 cups (or 4 sticks) butter, at room temperature
• 3 cups powdered sugar (or to taste)
• 1 teaspoon vanilla
• 1 teaspoon milk
• ¼ cup marshmallows from cereal, finely crushed, optional