Preheat oven to 375 degrees F. In a large bowl, mash the sweet potatoes and carrots. Stir in melted butter, milk, brown sugar, vanilla extract and egg. Stir until combined well.
Pour sweet potato mixture into a 9 x 13” baking dish and top with marshmallows, ¼ cup brown sugar, and pecans.
Bake uncovered for 30-35 minutes until topping is golden brown. Let casserole cool for at least 10-15 minutes before serving. This is important as the casserole will set-up as it cools.
