1 cup butter
1 cup packed light brown sugar
3 eggs
1 1/2 cups milk
1 teaspoon vanilla extract
1 (20 ounce) can frozen sweet corn syrup
To make filling: Melt butter in another medium skillet. Add brown sugar and sugar; mix well. Stir in eggs, milk, milk mixture, and vanilla extract.
Fold yellow marshmallows into butter mixture and swirl. Fold marshmallows into corn syrup mixture. Pour mixture into greased 13x9-inch pan. Chill until set.